A container ship carrying around 1700 tonnes of fuel has crashed into a reef off the coast of Tauranga, and is now stuck there, flooding.

Maritime New Zealand (MNZ) said on Wednesday afternoon some fuel from hydraulic pumps on the Rena, a Liberia-flagged 235m vessel, had leaked, but its fuel tanks were still intact.

Rena had been heading to Tauranga from Napier when it crashed into the Astrolabe Reef, about 7km north of Motiti Island, around 2:20am, according to Stuff.co.nz.

Maritime NZ incident controller Renny van der Velde told media at a conference the ship was stuck "hard and dry" on the reef, adding the national oil response team had been activated as a precaution. He noted there was "significant flooding" onboard in two of the ship's cargo holds and in two twin tanks to the side of the cargo hold.

"But the important thing is that everyone is safe," van der Velde said. There were 25 crew aboard, who would remain on the ship.

Port operations in Tauranga have not been affected.

New Zealand Dive and Salvage general manager Howard Saunders says Rena looks to be "well stuck."

"I thought when I looked at it before it certainly is a matter of weeks but it could well be that it's going to take a couple of months to handle it, it's not an easy job."

Wildlife experts from Massey University rushed to Tauranga to support the response team and manage any risk to wildlife.