Enough that a BAS or tax notice is never a shock — which means treating the GST you collect and the tax on your profit as money that was never yours to spend. A simple, safe habit is to sweep a fixed percentage of every deposit into a separate account, then true it up against your actual position each quarter rather than guessing once a year.
What it depends on
Where judgement stays human: The right percentage for your situation — and adjusting it as you grow — is a judgement based on your actual figures, not a number off the internet.
GST is the clearest case: the GST portion of a sale is collected on the ATO’s behalf, so if it sits in your everyday account it will feel like income and get spent. A separate “tax” account turns an abstract liability into a visible balance.
Income tax is harder because the right percentage depends on your structure and marginal rate, and on PAYG instalments once you’re in the system. That’s why a flat rule of thumb is a starting point, not the answer — it should be trued up to your real numbers.
The right percentage for your situation — and adjusting it as you grow — is a judgement based on your actual figures, not a number off the internet.
No question is silly, and there's no judgement about where things are at. Tell us what is different about your situation and a CPA will reply personally — usually within one business day.
Reviewed by Graham Chee FCPA (TPB registered tax agent, RAN 15375009) on 1 September 2026.
Sydney Bookkeeping · general information current as at 1 September 2026. This is general information only, not personal financial, tax or legal advice.