Auckland Council's pollution arm is investigating an illegal fuel dump after a truck mistakenly filled up with petrol then poured it down a stormwater drain.

The incident took place outside a North Shore service station on the corner of Rosedale Rd and Apollo Dr around 3.30am yesterday, when a truck driver emptying his tank into the street after he filled his diesel-powered vehicle with 91 unleaded petrol, not realising it was the incorrect fuel.

An eyewitness told the Herald he watched the driver fill up with petrol before driving out of the BP service station and dumping the fuel into a nearby stormwater drain before returning and filling up with diesel.

Fire and Emergency northern shift communications manager Paul Radden said firefighters were called to mop up the polluted waterway early yesterday morning.

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But by the time firefighters arrived there was little they could do with most of the fuel already flushed into the drain.

"Upon arrival there wasn't much we could do as most of the fuel had gone into the drain due to the delay in getting the call," said Radden.

As a result firefighters were unable to absorb or contain the fuel.

Auckland Council had also been advised about the incident.

Council compliance manager Steve Pearce said officers were now investigating a possible environmental breach.

Pearce said the council was notified about a pollution incident early yesterday morning over oil leaking down a drain on Rosedale Rd.

"We are currently investigating and if there is evidence of a deliberate discharge of fuel or oil into the stormwater system, we will be considering enforcement options and recovering any costs for the clean-up," said Pearce.

The council was not able to say which company the truck belonged to while the matter is under investigation.

A BP spokeswoman said it was a serious incident that occurred away from its premises and the company would co-operate with the council investigation.

"We don't condone the behaviour, the environment is very important to us."

She said BP had protocols in place for motorists who filled up with the wrong fuel, including helping them to refuel or organising a tow if needed. In this case, the motorist did not raise the problem with staff at the station, she said.