Truck GVM

I'm not certain Gordon but I don't think so. I expect someone else will post to let you know. I'm just posting to make the point that if you are planning to use the truck as a camper that de-rating it's GVM will make life very difficult for you - it will be a struggle to stay within the GVM. Getting a light Rigid licence is very easy, far easier than getting a car licence..... they assume you already know how to drive! :)

We last did this exercise in 2003 with a 3500 Canter so I’d assume it has changed, but the GCM was only derated if you made changes to the truck for its GVM derate which would affect its ability to tow. I would strongly suggest you ask an accredited vehicle engineer this question with regards your specific truck though.

Is it a Isuzu NPS300 (Early 6000kg GVM & later 6500gvm & new up to 7500kg) & you what it to be a NPS250 (same truck different GVM) EG: if yours is a NPS300 (6500kg) with a CVM 10000kg it will go down to a CVM of 8000KG but be wary if you go down in your GVM you want to check how heavy it is NOT buy its rego papers but buy a weigh bridge ticket it does not take much pending tray, spare wheel tool boxes etc to go over your 4500kg rating
& you can still get pulled in to a checking station if the police or pending state buy Gov transport Dept. you might notice a lot truck checking stations have already changed to checking from 4500kg & are doing random car & caravan, 4x4 & campers etc as most are over there GVM or the CVM
Good Luck

Just the way I see it:
GCM cannot be greater than GVM + BTC.
So unless you were somehow able to increase Braked Towing Capacity, you're GCM will be de-rated.