Theorem.

Big Business Expertise, Small Business Heart.

and advice you can actually trust.

About Theorem

With over two decades of hands-on experience spanning transport and logistics, agriculture, allied health, and the start-up space, Theorem offers practical, grounded support that meets business owners where they are — and helps them move forward with confidence. Theorem is more than just an accounting firm. We are strategic partners, helping small to medium businesses and startups optimise operations, streamline processes, and gain financial clarity that drives smart decisions.

Founded by business transformation specialist Cerissa Marshall, a Chartered Accountant, Registered BAS Agent, Six Seconds EQ Practitioner, Dare to Lead™ Trained, and is a graduate of the INSEAD Management Acceleration Program in Business Administration and Management, Theorem brings deep expertise and a sharp eye for detail. 

Cerissa’s extensive background, from grassroots bookkeeping at age 17 to managing billion-dollar mergers, gives Theorem a unique understanding of business challenges from all angles. Alongside her technical expertise, Cerissa has a true passion for mentorship, helping business owners become stronger leaders and decision-makers.

At Theorem, we use what we call the “hawk and mouse” approach - seeing both the big picture and the crucial details. We help business owners cut through the noise, solve real problems, and create sustainable growth by focusing on the right strategies at the right time.

Cerissa’s passion for small business owners fuels Theorem's mission. Our approach is calm, solutions-focused, and deeply human, ensuring business owners feel supported while making confident, informed decisions. Theorem is not just about the numbers - we’re about empowering business owners with the right tools, knowledge, and connections to succeed.

Our Mission & Vision

We believe that business success isn’t just about numbers: it’s about people, processes, and performance. Our mission is to be the trusted advisor and sounding board business owners turn to when they need clarity, confidence, and practical solutions that reduce stress and drive results.

We don’t just get the daily nitty gritty done, we empower business owners with proactive financial strategies, operational efficiencies, and a clear path forward. Whether it’s cash flow challenges, systems optimisation, employee issues, or scaling for growth, we provide the insights and support needed to steady the ship and move forward with confidence.

“My biggest challenge was knowing the importance of the things I wanted to implement for automation, but struggling to marry it all up. I think I always felt like a bit of an imposter. I owned a business but it wasn't running like a business.

Cerissa has been a big asset in this regard. She’s been brilliant at solving the hard parts and bringing new ideas to the process. I would call her the queen of common sense... a bookkeeper with a mathematical, analytical mind is rare to find.  

Now, our new business automation processes mean I have a real trust in our systems.“

Joe Henry

Owner, The Podiatry Room

“We didn’t have the capacity and financial acumen to successfully apply for and acquit grants. Our accounts were not organised in a way that allowed us to accurately forecast and undertake annual planning.

After working with Cerissa, our grant income has increased as has our ability to acquit grants appropriately. We have peace of mind knowing the business has reached a level of professionalism which is reflected in our ability to grow and achieve our goals.

Cerissa was easy to work with and very knowledgeable. She had great local knowledge and a well-rounded understanding of our industry. 

Nothing seems like a challenge now that we are working with Cerissa: She is actively helping the business to achieve goals and continue to grow.”

Belinda Tatipata

Executive Officer, Deadly Enterprises

How We Help

Strategic Business Advisory

• Helping business owners make informed decisions with clear financial data and actionable insights.
• Business process reviews and efficiency optimisation to maximise internal resources.
• Identifying cash flow bottlenecks and creating sustainable financial solutions.

Business Mentoring

• Acting as a sounding board for business owners navigating challenges in leadership, Human Resources, systems and financial decision-making.
• Providing practical, real-world insights that go beyond traditional accounting.
• Leveraging an extensive network of industry specialists to connect clients with the right resources.

Finanical & Operational Optimisation 

• Budgeting, forecasting, and financial modelling to support growth strategies.
  Corporate governance, compliance, and risk management.

• Basic bookkeeping, BAS agent duties, and accountancy services to ensure financial accuracy and compliance.

• System integration and automation to streamline operations, improving efficiency and reducing manual tasks.

Past Projects Include

Allied Health Provider 

Annual Turnover $400 000

Implemented a system that seamlessly integrated with client management software, automating new patient intake and post-appointment review requests. Streamlined operations by sending patient details forms in advance, enhancing efficiency, client experience, and retention. Optimised resource utilisation to support sustainable growth.

Remote Health Services

Annual Turnover $1M

Advised the founder of an allied health centre servicing remote communities across the Top End. Worked closely on business strategy, operational planning, and financial systems to support sustainable growth as the organisation scaled. Provided ongoing support as a trusted sounding board through the early growth phase, helping build strong foundations for long-term impact.

Regional Agriculture Business

Annual Turnover $10M

Partnered with a family-run agricultural operation navigating complex HR and cash flow challenges. Supported a shift in accounting structure to improve clarity and reduce stress, while providing ongoing advisory on employee management, payment systems, and financial forecasting. Helped move the business from reactive decision-making to proactive, calm, and strategic planning.

Australian Transport Company

Annual Turnover $18M

Consolidated group reporting to include weekly dash reports, monthly management accounts, board packs and business performance analysis. The new reporting allowed for group-wide financial performance reporting and analysis to be provided to the Group Country GM and Finance GM to guide post integration decision-making.

Leading Multinational Oil Company

Annual Turnover $3.07 billion

Consolidated group-wide financial reporting, providing executives with real-time performance insights. This streamlined post-merger decision-making, optimising resources and operational efficiencies.

Work With Us

• We Get It – We’ve worked inside businesses from the ground, up to multi billion dollar projects, not just outside as advisors. We know what truly matters.
  
• Beyond the Numbers – Our expertise extends beyond accounting into business strategy, optimisation, and leadership support.
• A Trusted Sounding Board – Clients value our straightforward advice, our ability to simplify complexity, and the calm, solutions-focused approach we bring to every challenge.
• We Speak Your Language – No confusing jargon - just honest, practical insights that make a real impact.
• Connections That Matter – When your business needs specialists beyond our scope, we connect you with the right people to move you forward.
  

At Theorem, we understand that every business faces unique challenges, so the first step to begin our partnership is a discovery call with Founder Cerissa. This helps us understand your specific challenges and explore potential solutions. 

During this call, Cerissa will assess your current situation and work with you to determine the best path forward.

Please fill out the contact form below, and Cerissa will be in touch to arrange a convenient time for your call.

"Cerissa’s skills and experience in finance at a senior level means she is a trusted and valuable business partner.

Beyond her professional experience, Cerissa displays a strong moral compass in the way she conducts herself. This is demonstrated in not only her professionalism but the strong, positive connections she maintains with those in her networks."

Kim West

Financial Controller

 “There are very few people in this world that just stop me in my tracks, and you are one.

You are simply extraordinary. And by simply sharing your space you elevate me as a leader and for that I say thank you."

Drew 

Managing Director

Theorem | Develop. Transform. Renew.

📞 0413 334 085
📧 cerissa@theorem.com.au
🔗 Connect on LinkedIn

Liability limited by a scheme approved under Professional Standards Legislation