Not sure about the regs in the Northern Teritory but I imagine they must be much the same as everywhere else. The ADRs are a federal thing and I imagine all of the states would have to keep pretty much in line with them.
There must be a roads corporation like VICROADS that looks after all of the regestrations and licencing. They should have an engineering department that would be able to tell you what you need.
Imagine owning a cobra in the only Aussie state with no speed limits..... I'd watch out for those road trains though. I was up there about 8 years ago and passing one of those going 100mph was fairly nerve racking.
Check out some of the cool number plates up there.
http://www.geocities.com/ausplates/NT/NT1st.htm
I want the cannonball plate!
Cheers