Your argument is flawed and you won't win it on the basis of that sign.
The sign is placed at the boat ramp and is supported by the solid yellow line along the kerb you can see in both photos, all of which denote 'no stopping' and is backed up in the City of Gold Coast parking bylaws (look them up, I just did). The exemption in the sign is for cars with boat trailers at the boat ramp, which presumably covers the short time required to stop and prep the boat for launch and pull the bungs out after recovery.
You're not using that sign to justify stopping with the 'boat trailer' at the ramp, you're trying to use it to try and park in the adjacent carpark, that won't work.
Elsewhere in the CoGC parking bylaws it does refer to the definitions of;
motor vehicle means a vehicle propelled by a motor that forms part of the vehicle, and includes a trailer attached to the vehicle.
trailer means a vehicle that is built to be towed, or is towed, by a motor vehicle, but does not include a motor vehicle being towed.
boat means any kind of boat or other vessel used or intended to be used, in navigation by water or for any other purpose on water and includes a boat or other vessel ? (a) whatever its size; and (b) however it is propelled or moved.
So, my take on that is that if the parking bays near the ramp are over-sized to accommodate vehicles with boat trailers then you might get away with parking there with a trailer if you abide by the signs and pay applicable fees, that's if you think your kayak, windsurfing rig or kite gear fits the definition of a boat.
You're best bet is to find the definition of a vessel in your state Marine and Harbours legislation, I'm sure that's going to be liberal enough to accommodate what you need.
So no, I'm thinking your argument is flawed and you wouldn't win but spending just ten minutes on google from the other side of the country with nothing but the information in those photos (I didn't even know what state Labrador was in ten minutes ago) has provided you a better argument to work with. Google is your friend, most of the time what you seek is there if you look in the right place.