GHB and other related substances are classified as Class B drugs in New Zealand. The maximum penalty for importation/manufacture/supply is 14 years imprisonment, and for possession 3 months jail and/or a $500 fine.
It is illegal to drive a motor vehicle while under the influence of any drug, including GHB. Breaking this law carries heavy penalties including disqualification from driving, fines and even imprisonment. It is advised that you do not drive after the consumption of any mood altering substance. You could put your own and other people's lives at serious risk.