More than 500 drivers have tested positive for drugs since the regime began two years ago, the government says.

It has reviewed the law - an undertaking given when it was introduced - and says it isn't going to be changed.

"The law is helping take these people off the roads and reducing the risk they pose to themselves and other road users," Associate Transport Minister Simon Bridges said on Tuesday.

The government has decided not to introduce random roadside drug testing.

Mr Bridges says saliva tests aren't sufficiently reliable.