By Emma Hatton, RNZ

Electric vehicle owners are being warned to top up if heading over Transmission Gully, because the steep road will drain their batteries faster than the old coastal route.

Laura drove her Nissan Leaf from Waikanae to Wellington earlier this week and said because of the gradient of the road as well as the wind buffeting the vehicle used more power than when she drove the old route around the coast.

"When I drive to Wellington I get there with more than 60 per cent battery at the end, but [over Transmission Gully] I arrived to Wellington with 52-53 [per cent], which is quite a big difference."

Laura said she previously did the round trip to work and back on a single charge, but she would not risk that now.

She was surprised there was not a rapid charging station in case people - particularly out-of-towners - got caught short.

"One on each side - that would make you feel safer that you can take Transmission Gully without any hiccups. Especially because it's a direction that everybody is coming towards - if we are going to have a preference on electric vehicles, it's nice to have the infrastructure behind it as well."

Motorists heading south who need to charge their batteries have to pull off State Highway 1 and top up in Paraparaumu, and for those going north the last chance is at Churton Park, or to head in to Porirua and then backtrack.

Automobile Association motoring affairs general manager Simon Douglas said the new road would be hungrier due to the hills, and drivers needed to be prepared.

"Find out where the available chargers are along the route and if you're entering portions of the journey where you're nervous about coverage - top up before you get to those parts."

He said most trips people took in an electric vehicle were local, but highway investment needed to ramp up for longer journeys.

"As more EVs enter the fleet there's going to be more competition for those spaces so we definitely need to see more investment in different parts of the network to cater for the types of trips people will be taking."

A spokesperson for EV charging stations provider ChargeNet said it was talking with lines companies about where the best spots might be for charging stations near, or on, Transmission Gully and other highway stops around the country.

A Waka Kotahi New Zealand Transport Agency spokesperson said people should not be stopping on the motorway unless in an emergency, adding there were steeper and longer sections of the state highway network in other parts of the country.

Ventia, the company that manages the road, did not respond to RNZ's request for comment.