but ...
according to https://infrastructure.gov.au/roads/..._b.aspx#anc_11
The Gross Trailer Mass, (in this case 1900 kg) must be distributed "approximately uniformly distributed over the load bearing area" -- it the boat and trailer is a concentrated load resting where the boat trailer's wheels are sitting.
BTW, I think by making the modifications he has, GrumpyJ's "protagonist" has become the manufacturer and needs to get a new compliance plate for the trailer.