A "suspicious fire" at a house in Pokeno, south of Auckland, today has police investigating a possible P lab.

Fire northern communications shift commander Nicole Bernard said firefighters were called to a three bedroom house in a rural area in Helenslee Rd at 4pm.

The fire was "well involved" when crews arrived, she said. "The building almost collapsed."

Counties Manukau Police spokeswoman Kimberley Mathews said a clandestine laboratory team - used to investigate possible P labs - was at the scene.

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"It cannot be ruled out that the house was previously used to manufacture drugs," she said.

It is not yet known if anyone was home at the time of the fire.

More information will be available following the scene examination, she added.