Three helicopters and multiple fire trucks are being used to battle a large spreading fire at Kaipara Flats near Warkworth.

Firefighters were called to a vegetation fire about 10am and initially responded with two appliances, Fire and Emergency NZ spokesman Simon Lyford said.

On the way, they called for two tankers to join them as they believed they might run out of water.

On arrival they realised they were dealing with a "large" fire and that the terrain would make access difficult, Lyford said.

As they battled the blaze, they called for further aid from more appliances and from helicopters.

"We have a number of appliances there on the ground as well as three helicopters working," Lyford said.

He could not yet provide an estimate of the size of the fire, but said efforts to put it out were hindered by the terrain and wind.

Meanwhile, a 150m by 50m vegetation fire in a paddock in Albany is being treated as suspicious.

Firefighters are still working to extinguish that fire, which they were called to shortly before 2pm.

They have asked for a fire investigator to attend.