When a BAS error produces a tax shortfall, the ATO does not apply a flat penalty. It applies one scaled to the behaviour that caused the error, and then it scales that again based on when you told them. Those two levers, culpability and timing, are what actually determine the cost, and the second one is almost entirely within the firm's control. Understanding how they interact is the difference between a correction that costs almost nothing and one that costs a meaningful fraction of the shortfall, which is why voluntary disclosure is one of the highest-leverage decisions a firm makes after finding an error.
How the ATO sets the penalty, and why timing changes everything
The base penalty is a percentage of the shortfall amount, not the total tax, and the percentage tracks the behaviour behind the error. Failure to take reasonable care attracts 25%. Recklessness, meaning a reasonable person would have seen a real risk of a shortfall and the taxpayer disregarded it, attracts 50%. Intentional disregard of the law, the most serious, attracts 75%. So a $20,000 GST shortfall assessed as recklessness generates a $10,000 base penalty before anything is reduced, and it can also be loaded a further 20% in aggravating circumstances such as obstruction or a repeat shortfall. That is the ceiling. The floor is set by disclosure. A voluntary disclosure made before the ATO notifies the client of a review or audit reduces the base penalty by 80%, which turns that $10,000 into $2,000. The same disclosure made after the ATO has already made contact generally reduces it by only 20%, though the Commissioner retains discretion to allow more. And where the disclosed shortfall is under $1,000 and the disclosure is made before any ATO examination, the penalty is reduced to nil entirely. The gap between disclosing before contact and after is the whole ballgame, and it closes the moment the ATO issues a notice for that period.
There is one more distinction worth holding onto, because it protects the client but not the firm. A client is generally not liable for a false or misleading statement penalty where the error was caused by their registered tax or BAS agent failing to take reasonable care, provided the client gave the agent all the relevant information. That safe harbour is real and worth knowing, but it shifts exposure onto the firm rather than eliminating it, which is precisely why a firm that finds its own error in a client file has every incentive to correct it fast rather than hope it goes unnoticed.
What a voluntary disclosure actually is, and how to make one
A voluntary disclosure has to be made in the approved form and has to genuinely tell the ATO about the shortfall or the false or misleading nature of the original statement. For GST corrections the most common method is simply lodging an amended BAS through the accounting software, and outside a review or audit the client can also disclose through ATO Online Services for Business or in writing via secure mail. A crucial and reassuring point, confirmed in the ATO's own guidance, is that the client does not need to admit liability or concede the original statement was wrong. They only need to identify that there is a correction to make. What matters is that the shortfall is quantified clearly, the relevant period is named, and enough detail is provided for the ATO to assess it without launching its own examination. A vague "something may be off" is not a disclosure; a specific, period-identified, quantified correction is.
The one rule that governs all of this is timing. The 80% reduction disappears the day the ATO tells the client it will examine that period, so every day an identified error sits undisclosed is a day the cheapest option is quietly expiring. A firm that discovers an error and waits to see whether the ATO notices is gambling with the client's money, and the odds get worse, not better, with time.
The takeaway
A BAS error found after lodgement is fixable and often cheap. The same error found by the ATO first is significantly more expensive, and the entire voluntary disclosure framework exists to reward getting there first. Because the pre-contact reduction is so large, the cost of disclosing almost always sits well below the cost of not disclosing and being found. When an error surfaces, the sequence is straightforward: quantify the shortfall, prepare the amended BAS or written disclosure, and lodge it before the ATO has any reason to be looking at that period. The firm's real job is to make sure the client understands how much the timing is worth, and then to move quickly, because the single most valuable thing in this entire process is disclosing one day before the ATO does, not one day after.
