The info on "Interstate or overseas vessels in NSW" at
www.nsw.gov.au/driving-boating-and-transport/boat-and-vessel-registration/interstate-and-overseas-vessels suggests the maximum is three months.
To use NSW waterways, you
don't need NSW registration as long as your vessel:
- is registered in another state, territory, or overseas
- isn't normally used on NSW navigable waters
- hasn't been in NSW more than 3 months
- is properly numbered, and
- carries an identification plate required by the vessel's home state, territory or country.
It's the same in Tassie, for example. Not sure about other states and the NT.
I suspect it is a pretty standard requirement (similar to requirement to locally register a car that will be garaged in a state for more than 3 months).
I'm not sure how actively policed it is. If you're on a swing mooring, the local boat safety officer may make a note of comings and goings in his/her area. If you're in a marina, I doubt NSW Transport keeps a close eye on it - though I s'pose it's possible they ask marina operators to report on it. Or marina operators may keep a check on it 'cause it's a requirement in their insurance, for example. But I'd be surprised if they do.
It might pay to make some discrete inquiries of a couple of marina operators about berthing your boat for 6 months before heading back to Queensland and see if they raise it as an issue.
Mind you, the cost of NSW registration is likely to be only a fraction of the cost of a marina berth for 6 months.