By Joshua Occhino, Co-founder, Ezyiah
Short answer: You can generally amend a BAS within four years. The ATO's period of review starts on the day the BAS is lodged and ends four years and one day later. But how you fix it depends on the error. A credit error (you paid too much GST) can be corrected on a later BAS lodged within that four-year period, with no value limit. A debit error (you paid too little) can only be corrected on a later BAS within 18 months of the original due date if your GST turnover is under $20 million (12 months if it's $20 million or more), and only within value limits. Otherwise you revise the original BAS. Separately, unclaimed GST credits expire four years after the due date of the earliest BAS you could have claimed them in.
Which time limit applies to my error?
| Error | How to fix | Time limit |
|---|---|---|
| Credit error (overpaid GST) | Correct on a later BAS | Within the period of review: 4 years and 1 day from lodging the original BAS |
| Debit error (underpaid GST), within limits | Correct on a later BAS | 18 months from the original due date (turnover under $20m), or 12 months ($20m or more), plus value limits |
| Debit error outside the limits | Revise the original BAS | Within the period of review |
| Unclaimed GST credits | Claim on a later BAS or revise | 4 years from the due date of the earliest BAS the credit could have been claimed in |
Sources: ATO types of GST errors, period of review and time limits on GST credits and refunds.
What's the difference between a credit error and a debit error?
- Credit error: you reported or paid too much GST. Examples are overstating GST on sales or reporting a sale twice.
- Debit error: you reported or paid too little. Examples are understating GST on sales or claiming a GST credit twice.
You can only correct a debit error on a later BAS if it's within the time limit and the value limit for your current GST turnover, wasn't caused by recklessness or intentional disregard, and isn't in a period the ATO has told you it's examining. The value limits are set out on the ATO's types of GST errors page. Check them before rolling a debit error forward. If it's outside the limits, you need to revise the earlier activity statement, and general interest charge may apply to the underpaid amount.
Why is the four-year GST credit limit a trap?
The credit time limit is separate from the period of review. It generally runs four years from the due date of the earliest BAS in which you could have claimed the credit. Lodging a revised BAS doesn't stop that clock, so if a client is close to four years on an unclaimed credit, act early. The same kind of limit applies to fuel tax credits. See GST credits: the four-year expiry.
How do you work out whether you still have time?
- Classify the error as a credit or debit error.
- For a credit error, confirm you're within four years and one day of lodging the original BAS, and check that any GST credits involved are still within the four-year credit limit.
- For a debit error, check the 18-month or 12-month limit from the original due date, and the value limit. If either is exceeded, revise the original period.
- If there's a shortfall, consider a voluntary disclosure to reduce penalties.
See also the ATO's guide to fixing BAS mistakes or making adjustments.
How Ezyiah helps
Ezyiah is an AI bookkeeper with its own double-entry ledger. It reconciles live bank feeds daily and builds the BAS from the same books, and the Ezyiah Assistant panel and activity logs show what changed and when. Errors are easier to find during the quarter, and when you do have to correct a past period, the transactions and changes behind each figure are there to check. Your accountant reviews and approves corrections before lodgement.
See Ezyiah for accounting firms and for business owners.
Frequently asked questions
Can I amend a BAS after 4 years?
Generally no. Once the period of review has ended, the assessment usually can't be amended, except in limited cases such as fraud or evasion.
Can I fix a BAS mistake on my next BAS?
Often, yes. Credit errors can be fixed on a later BAS within the period of review. Debit errors can only be fixed this way if they're within the time and value limits.
Does lodging a revised BAS stop the GST credit time limit?
No. The four-year credit time limit keeps running.
Is there a penalty for amending a BAS?
Correcting a genuine mistake isn't penalised in itself, but general interest charge can apply to underpaid GST, and shortfall penalties may apply. A voluntary disclosure reduces them.
