Eight people have been taken to hospital and 50 more are being assessed after a gas leak at a building in downtown Auckland.

People staggered out of the building like "zombies" as occupants were being evacuated and it was frightening to watch, a witness said.

A police spokesperson said they were called to a building in Victoria Street West by Fire and Emergency at 9.35am to assist with cordons.

Emergency services were responding to the effects of what was believed to be an unknown chemical substance, which forced the evacuation of the building.

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Eight patients have been taken to Auckland Hospital in a stable condition - some were vomiting and complaining of chest pains.

Fifty more people needed to be assessed and a triage area has been set up outside the Augusta House building at 19 Victoria Street West.

A Fire and Emergency spokesperson said it had discovered that a smell of fumes was coming from the air conditioning system. Staff were working with contractors to find the source of the problem.

Firefighters were investigating to ensure that "it's not causing any harm to anybody".

Ambulances and a fire truck are at the scene.

There is a Countdown supermarket near the site.