well from my experiences
2.2m beam is the most you can legally tow a boat on the road (as with trailer it come out to 2.5m), however, there are a few that can get away with it
2.5 max width for your boat, with trailer it comes close to 3m, which is considered illegal once u exceed 2.6m, therefore, all insurance on car and boat is voided,
2.6m - 3m u need oversize signal and flashing yellow, but still no insurance lol
3m - 3.5m u need to apply for road escort, but with insurance
others rule is if it exceed 19m in length that illegal already and there no way around that... for a boat with 3m... I take it your towing something of 8m that weight close to 4.2T.... hope you're using a tipper
remember you can techy legally tow a boat if it fit on one side of the road, however, good luck taking that up with insurance if the worst come to be
hopefully, that give you some inslight what you're seeking for, otherwise, TMR will refer you to
http://www.tmr.qld.gov.au/-/media/bu...lers.pdf?la=en