If your business turns over more than $75,000 a year, you need to be registered for GST. That's not optional.
Once you're registered, you collect 10% GST on top of your prices and send it to the ATO, minus any GST you've paid on your own business expenses. The difference is what you owe. That's the whole system.
Where people get into trouble is treating the GST they've collected as their own money. It isn't. Ten percent of every invoice you send belongs to the ATO. If you spend it, you'll have a problem when your BAS is due.
The simplest fix is to open a separate account and move 10% of every payment you receive into it automatically. When your BAS comes around, the money is sitting there. No scramble, no stress.
BAS lodgements are usually quarterly, March, June, September, and December. Missing them attracts penalties and interest, and the ATO notices patterns. If you're consistently late, you'll eventually get more attention than you want.
If you're close to the $75,000 threshold, register before you cross it, not after. Backdating GST obligations is a mess.
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