Fire crews at the Port of Tauranga are waiting to see what happens when the hatch covers of a ship that's on fire come off shortly.

There's a blaze below decks on Hong Kong-registered Kokopo Chief, which has 650 tonnes of timber on board.

Bay of Plenty coast area commander Murray Binning says a decision's been made to lift the hatches around 10am.

He's hoping firefighters will find a very small fire and put it out quickly.

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But he says once the hatch covers come off, there's potential for the fire to flare-up.

However, he's confident crews can deal with that, using the equipment they've got there.

The northern fire communications centre Jaron Phillips said two fires started overnight in the hold of the ship while it was berthed at the Port of Tauranga.

It wasn't clear yet what had caused the fires.

The cargo ship had crew on board and fire had been liaising with the captain this morning, Phillips said.

He had not been told of any injuries.

"It's not a situation where crew are trapped below deck," he said.

Seventeen fire trucks were attending.

Seventy firefighters were at the scene, including from stations in Tauranga, Greerton, Mount Maunganui, Papamoa, Te Puke, Omokoroa, Rotorua, Avondale, Hamilton, Maketu and Katikati.

A specialist breathing apparatus tender had been mobilised from Auckland.

All 48 containers on board the ship had been removed shortly before 6am and two aerial appliances were set up with fire crew on a tugboat ready to remove the ship's hold cover and check the extent to which the fire had spread.

The alarm was raised following indication of high temperature in the hold of the ship.

The Kokopo Chief is registered as having a Hong Kong flag and was built in 1991.

It was just over 158m long and 22.2m wide at its widest point.

A Herald photographer at the scene said it was difficult to see anything happening within the port, which was surrounded by fences and security.

He said he had spoken to several boat workers at the port but none had seen anything this morning.