A Kaiwaka petrol station that State Highway 1's heavy stream of traffic usually goes straight past has nearly doubled its sales since the fuel tax hit the Auckland pumps.

Within hours of the 11.5 cents a litre Auckland region-wide tax being introduced, Shamrock Service Station, 7km north of the Auckland/Kaipara District border, clocked about 150 per cent more custom than usual.

The Caltex station usually sold more fuel to local traffic than the potentially lucrative, but already tanked-up, SH1 traffic.

Yesterday, manager Manpreed Kaur described a ''very busy two days'' since the tax was introduced on Sunday.

''Things are changing right now. We do not know for how long,'' she said.

''But we are the first petrol station after leaving Auckland and the last before going into Auckland.''

Shamrock's 91 unleaded petrol was selling for $2.09 a litre yesterday.

Just south of the border at Wellsford, less than 20km from Kaiwaka, a service station manager said the toll from the Auckland fuel tax had been felt immediately.

He said it was against company rules to give fuel prices over the phone, but confirmed it was at the new Auckland price, and ''we have already suffered".

A spokeswoman for Z Energy said the petrol company was monitoring site volumes closely, but did not expect to draw any firm conclusions around the magnitude of any across-the-border buying for at least a week.