An unconscious man has been found in a critical condition in Auckland Domain overnight.

The man, aged in his 20s, was this afternoon in a stable condition in Auckland City Hospital, an Auckland District Health Board spokeswoman said.

A section of Domain Dr has been closed to traffic as police examine the area where he was discovered at around 12.45am.

"At this stage it is unknown how the man received his injuries and police are speaking to people who were in the area," police said in a statement.

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Detectives from Auckland City Crime Squad are conducting the enquiry.

The man was initially assessed as being in a critical condition when found, police said, but it is believed he had since improved slightly.

A blue crime scene tent has been erected on Domain Drive on the road alongside the Duck Ponds.

It's cordoned off at either end, with a police guard on both sides.

Officers are turning away runners and people walking their dogs.

The area around the Wintergarden Pavilion, including the cafe's outside dining area, has been cordoned off by police, and is guarded by a police car.

The main road through the Domain, from the Grafton entrance at Auckland Hospital is also cordoned off, however access to the pitches an the museum is open.

Early morning walkers and runners are still using the park, despite the rain and crime scene nearby.

Road closures will remain in place until at least 10am, police said.

Motorists are asked to avoid the area.