Four birds have been found dead in an oil slick spanning 2km from the container ship that crashed into a reef off the coast of Tauranga, north of New Zealand.

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

The Rena has been at the centre of a big salvage and environmental management effort since it struck the Astrolabe Reef north of Motiti Island about 2.20am Wednesday.

Maritime NZ incident controller Renny van der Velde told a news 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.

Representatives of the salvage company boarded the vessel and confirmed the leak had stopped and only a small volume of oil had gone into the water, the New Zealand Herald reported.

Relevant specialist groups, including wildlife experts, have responded to the situation.

"While reports of a leak are unfortunate, they are not unexpected. We were lucky in the sense that we had all of yesterday to get the team to Tauranga and get plans, equipment and people in place," National On-Scene Commander Rob Service told the Herald.

"We are ready to launch whatever level of response the situation requires."

Astrolabe Reef is home to an abundance of marine and bird life including petrels, little blue penguins and a seal colony. It is a popular spot for fishing and scuba diving, with drop-offs of nearly 40m. Maritime New Zealand said none of those locations were believed to be affected.