I'm not entirely convinced that a regional petrol tax is the way to go: I think a targeted congestion charge for the CBD and major arterials might be a better way of fighting congestion while funding public transport. It also seems fairer to concentrate the charging on places where there are good public transport options.
Whichever way it happens, I'm sure the usual lobby groups will complain that motorists in New Zealand are over-taxed. They should have a look at this chart from the Ministry of Economic Development:
Our petrol taxes, and thus prices, are actually the fifth lowest in the OECD. So while New Zealand's dispersed population, supposed "pioneer" culture and "love affair with the car" are often used to explain our high levels of car use, there's another possible explanation: it's so damn cheap! Especially so when our public transport is required to regain a comparatively high proportion of its costs from fares.
An extra 10 cents a litre would actually hardly make a difference by international standards, and it certainly doesn't compare to last year's petrol price rises, which led to a 9% decrease in cars entering the CBD. Now that we're finally making baby steps towards funding a half-decent transit system, perhaps it is time to consider a pricing regime that makes public transport a more attractive option.