WRITTEN OFF: A police officer watches as a police motorcycle is destroyed by flames.

A police motorcycle burst into flames just moments after a police officer noticed smoke coming from under the seat.

The officer had stopped a car in Moorhouse Ave today and arrested the driver for possession of drugs and a BB gun.

He then got on his bike and started to move off when he noticed the smoke.

DAVID HALLETT/Fairfax NZ DESTROYED: A firefighter dampens down a police motorcycle after a fire engulfed it on Friday.

He pulled over and stepped back, just in time to watch the motorcycle become engulfed in flames.

The officer was not injured but the bike was "totally destroyed", a police spokesman said.

The fire happened about 7.40am at the corner of St David St and Moorhouse Ave and caused traffic to be delayed.

Paul Mills BIG BLAZE: The motorcycle was "well ablaze" when fire crews arrived.

A Fire Service spokesman said the bike was "well ablaze" by the time crews arrived.

"We've had reports coming in on a building on fire in the area, so that gives you an indication of how much smoke there was," he said.

Christchurch City fire station officer Kerry Richards said: "It was quite a fire. It was definitely well involved."

Police said the cause of the fire was unknown, but it was most likely a fuel leak or mechanical fault.

The motorcycle, a 2007 BMW R1200 RTP, has been removed and will be subject to a mechanical inspection.