A section of State Highway 20 in Auckland is closed this morning after a serious crash injured three people, one critically.

The southbound lanes are closed between the Cavendish Dr on-ramp and the Lambie Dr off-ramp, in Manukau, a police spokeswoman said.

The 5.10am crash involved two cars and one person had been critically injured.

Two other people were also hurt - one seriously, the second moderately, she said.

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The Serious Crash Unit was on the way to the scene just after 6am.