Emergency services are at the scene of a serious crash in Auckland's Mt Wellington which has left a person in a critical condition.

The crash happened at the intersection of Penrose Rd and Barrack Rd.

A St John spokeswoman said one person was being taken to Auckland City Hospital in a critical condition.

Police believe the person may have suffered a medical event; however the Serious Crash Unit is investigating. Penrose Rd is closed between Ruawai and Barrack Rds while officers examine the scene.

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The ambulance service was called at 12.49pm, with two manager vehicles and one ambulance attending. Police were called around 12.50pm.

Auckland Transport asked drivers to follow the directions of emergency services and prepare for a detour. Bus services along the route would be delayed, AT tweeted.

A person at the scene told the Herald the crash appeared to be a nose-to-tail, with minimal damage to the vehicle in the front.

The second vehicle had sustained more damage, he said.

The road closure was causing issues for at least one truck-and-trailer unit which was too heavy to drive down the detour road, he said.