I have been looking at GVM upgrade suspension online, all I can find is it needs to be done by an approved business, and needs an engineers report. My question is 'is the engineers report' an lvvta cert? Or is it different?
The GVM upgrades that are approved are from Australian suspension manufacturers such as Pedders (www.pedders.com.au/kits/gvm-upgrades/). They provide kits for utes such as Mazda BT50s and these have Australian approval to raise the manufacturer's GVM.
NZTA accept the Australian approval and aside from new unregistered vehicles, LVV certification is required.
No it doesn't, the GCM remains the same - increasing the vehicle weight reduces the towing capacity correspondingly. A few years ago it was allowed in Australia but they stopped the practice.
Also note that you cannot increase the GVM over 3500kg as it will become a heavy vehicle. As far as I know NZTA have not approved any vehicle to change from light to heavy due to a suspension GVM upgrade - if you need a GVM over 3500kg and a greater GCM then you need a heavy vehicle.