There appears to be a lot of conflicting information on TradeMe. One suggestion is that you use the standard 10 metre 240 volt Campervan lead and add a short Campervan/NZ 3-pin plug and plug directly into the generator. Another seller states that this is illigal and suggests changing the 3-pin socket on the generator for a Campervan socket - I cannot see how this is any different. Also the "House Power Supply" adapter is not suitable as it is for indoor use only - do they produce a weatherproof version?
Your right the standard 16a to 10 amp 3 pin adaptor is illigal for a number of reasons. The simplist way around it is to have an IP rated 16 amp socket added to your generator.
You are also right in that the house hold adaptors are not IP rated for use outside but neither is the generator for other than fine dry conditions - unless you get an IP rated one.