Home built trailers need a weigh bridge cert for first rego in NSW as well. For trailers under 750kg gmv you don't need a weigh bridge cert to transfer trailer rego in QLD.
Before I built mine I went in and spoke with the girls at Mario trailers in Sydney. I wanted a few custom options (shorter drawbar, slightly narrower width, eye to eye springs, etc), the response was if I couldn't get what I wanted in any of their standard models or standard options they wouldn't make it.
I would guess most manufacturers are the same as Mario, they have a list of models and all specs and weights have been checked and approved by transport departments. You just order the model you want, they stamp that name on the plate and the transport guys are happy.
Now the issues with gmv on a lot of local and imported trailers these days will come down to the extras or options. It would be safe to say the manufacturer has registered their models as the bare bones box and axle. Tool boxes, water tanks, kitchen, tent/canvas, bed, drawers, etc will all be eating into your payload. That's why you're getting your trailer home and realizing you only have a couple of hundred kilos to play with.
The manufacturer has just made it as easy as possible for them to register their product.
