A person has been killed on board the cruise ship Emerald Princess at Port Chalmers wharf after a gas cylinder exploded.

A St John Ambulance spokesman confirmed a person had been killed in the explosion. It is understood the deceased is a crew member of the ship.

Otago Coastal area commander Inspector Jason Guthrie said the deceased was a man. No-one else was injured in the incident.

Cruise ship operator Carnival Australia's spokesman said their first reports pointed toward the explosion happening while work on the hydraulic launching system on a tender vessel was being conducted.

After the gas bottle exploded it flew off the ship, landing on the wharf about 5pm today.

People were on the wharf at the time but none were injured.

"There was a loud explosion on board and a gas cylinder appears to have exploded. It is a very serious incident on board the vessel," Port Otago chief executive Geoff Plunket said.

Maritime Union Port Chalmers-Dunedin secretary Phil Adams said he was working in the port's control room when he heard "this massive explosion''.

"I just heard this massive explosion and got up and I saw this pipe or a tube lying on the wharf. . . it was a gas cylinder, by the sounds of things, that had just blown straight off the ship and on to the wharf.

"Man, I've never heard an explosion like it in my bloody life. I'd say most of Port [Chalmers] felt it.''

He did not see a fireball, and there were no obvious signs of damage on the cruise ship, but emergency services remained at the scene.

His first thought, upon hearing the explosion, was ``terrorism . . . but it's certainly not that''.

"It's quite sad, really, that something like this has happened at our port.''

The cruise ship had been due to sail at 6pm, but had been delayed as passengers returning from the city were kept back from the wharf following the explosion, he said.

Some were now being allowed to board the vessel, but it was not yet known when it would depart.

The noise of the explosion garnered attention on social media with people reporting hearing the explosion from as far afield as Sawyers Bay.

"Sounded like a container fell,'' one commenter said.

Police are investigating on behalf of the coroner. WorkSafe New Zealand and Maritime New Zealand have been advised.