Why the PAYG instalment problem is not really a calculation problem
The ATO is trying to improve the system
The ATO is trying to modernise the PAYG instalment system through a newer software-based method.
The idea makes sense.
If PAYG instalments are meant to help taxpayers pay tax progressively during the year, the system should reflect current business performance as closely as possible.
Using more current data may improve accuracy. It may reduce the gap between what is paid during the year and what becomes payable after lodgment. It may also reduce the need for taxpayers to manually vary instalments.
All of that is positive.
But I do not think the main weakness in the PAYG instalment system is the calculation method.
The real issue is cash flow
Many businesses do not underpay instalments because the formula is wrong. They underpay because keeping the cash is commercially better than paying it to the ATO earlier.
That is the uncomfortable reality behind the system.
For many businesses, cash flow is not an abstract concept. It funds wages, suppliers, rent, loan repayments, and day-to-day operations. Holding onto cash longer can genuinely help businesses manage uncertainty and pressure.
And if the downside of underpaying instalments is limited, many businesses will choose to retain that cash for as long as possible.
That is not always about avoiding tax. Often it is simply rational business behaviour.
What the current system already tells taxpayers
This is the part that matters most.
The PAYG instalment system allows taxpayers to vary instalments. That is reasonable. Business conditions change. Forecasts can be wrong. Income can move significantly during the year.
But the current framework also sends another message.
It tells taxpayers there is room to underpay without meaningful consequences.
Under the current system, penalties generally only arise where instalments fall below 85% of the actual liability.
In practical terms, that means a taxpayer can materially underpay tax during the year and still avoid penalties altogether.
That matters because behaviour follows incentives.
If taxpayers know there is flexibility, and they know the downside is limited unless the shortfall becomes large enough, many will deliberately retain cash longer.
Once that becomes normal behaviour, the issue is no longer really about calculation accuracy.
It becomes a behavioural design issue.
The pressure this creates for accountants
This also creates a difficult dynamic between accountants and clients.
Many advisers have experienced clients pushing to reduce instalments simply to preserve cash flow.
The discussion is often not really about accuracy. It is about retaining cash for longer.
And where clients know there are limited consequences unless the shortfall becomes significant, advisers can end up under pressure to support aggressive variations even where the position is uncertain.
The accountant may be trying to apply reasonable judgement and maintain compliance discipline, while the client is focused on preserving working capital.
The softer the consequences, the harder those conversations become.
Better software does not necessarily change behaviour
This is why I am not convinced the software method alone changes the real problem.
A better calculation method absolutely helps taxpayers who are already trying to pay accurately. It may improve fairness. It may modernise administration. It may reduce frustration caused by blunt instalment amounts.
But many taxpayers are making a different calculation altogether.
They are not asking: “What is the most accurate estimate of tax during the year?”
They are asking: “How long can I keep this cash without serious consequences?”
That is not really a software problem.
It is an incentives problem.
Strong systems need both accuracy and consequences
Strong tax systems are not built on calculation methods alone.
They also depend on incentives, enforcement, behavioural expectations, and credible consequences.
If one part of the system is weak, taxpayers adapt around it.
If the ATO wants more tax paid during the year, improving the calculation method is only one part of the answer.
The other part is whether the consequences for deliberate or material underpayment are strong enough to influence behaviour.
Without that, there is a risk the system simply becomes more accurate for taxpayers who were already trying to comply properly, while behaviour for everyone else remains largely unchanged.
The real question
The PAYG instalment issue is not really about software or formulas.
It is about behaviour.
A prepayment system only works properly if taxpayers believe they are better off paying earlier rather than holding onto the cash.
At the moment, I am not sure the system fully achieves that.
And until that changes, many businesses will continue making the same commercial decision: Keep the cash longer and deal with the tax later.