Five people were taken to hospital after a gas explosion at a building site in Auckland's Wynyard Quarter this morning.

Four people are in a serious condition in Middlemore Hospital while one is in a moderate condition in Auckland Hospital.

The incident was reported to police about 6.25am in Halsey Street in Auckland's CBD.

Police at the scene of the incident on Halsey Street in Auckland. Photo: RNZ/Nick Monro

Multiple ambulances and fire trucks were at the scene.

Two fire investigators remain on site. They say it could be days before the cause of the explosion is determined.

The scene of explosion in Halsey Street, Auckland CBD on 26 August 2022. Photo: RNZ / Nick Monro

It is believed to an LPG bottle exploded at the building site. Bits of burnt clothing could be seen scattered around.

The building site is in a very busy part of downtown Auckland.

A St John Ambulance spokesperson has confirmed it was an explosion.

It said it sent 10 vehicles, including five ambulances, to the scene.

The scene of explosion in Halsey Street, Auckland CBD on 26 August 2022. Photo: RNZ / Nick Monro

Tuck Shop Bakery owner George Seaton said he first noticed the incident when ambulances started arriving.

"We smelled some fire, smoke. There was someone being attended to ... a lot of emergency service workers around them."