Be careful with Visa ending dates. Are you on an eVisitor? Those are valid for a year, but each individual stay is only for 90 days. This means that your expiration and transition to a BVA is typically at the end of those 90 days.
Now, if you go on holiday overseas before your 90 day clock is over and your BVA kicks in, and then come back into Australia you've reset the 90 day clock.