Fishtits

Constable in uniform - yes (I assume they were in uniform?)

has reasonable grounds to believe the rider is driving other than in accordance with a license - yes (it is for a Court to decide if the Officer's grounds are reasonable but it is assumed the Officer rightly or wrongly believes the bike to be unrestricted)

and has requested to see both the DL and counterpart entitling the rider to ride that vehicle and a suitable license has not been produced - yes (the Officer believes the bike to be unrestricted, therefore the rider must produce a valid license for an unrestricted bike)

then the Officer can seize the vehicle under s165 of the RTA '88.

Thats all the law says.... It doesn't say the Police have to prove it's unrestricted prior to seizure, the Officer just has to believe it's unrestricted.