A motorcyclist is in critical condition after colliding with a car on State Highway One in Northland.

Another motorcyclist who also collided with the southbound car at Three Bridges, north of Kawakawa, is in serious condition.

The crash occurred just before midday.

A landing site was being prepared to take the motorcyclists by helicopter from the scene.

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Diversions are being arranged through Paihia on State Highways 10 and 11.

Police expect the diversions to have a significant impact on travel times and result in congestion on the alternative route.

"Motorists are advised to delay travel where possible and to take extra care and be patient on the lengthy diversion."