The regulations are silent but the NSW guidelines are pretty clear, if you are going to leave a firearm in a vehicle - anywhere - then it’s in a locked box attached to the vehicle. And if I were you I’d remove the bolt as well, or if a shotgun or lever put a trigger lock on it. The legal test is, did you take all reasonable precautions to avoid it getting stolen. Fact of the matter is, if it gets nicked from your car you aren’t going to enjoy it but if all they had to do was smash the window to get it, you’ll struggle to convince anyone you took all reasonable precautions.
Speaking for myself though, I usually just take one gun. Call the registry but unfortunately they usually won’t give a definitive answer.
Probably best guide is this, note the bit about transporting cat A and B, and if you are leaving them unattended you need to behave as if they were cat D
https://www.police.nsw.gov.au/__data/as ... _Sheet.pdf