An aerial observation flight this morning has confirmed oil leakage from the cargo vessel Rena overnight has led to a light oil slick stretching 2km.

Maritime New Zealand says the oil leak is in a narrow strip seeping away from the ship grounded on the Astrolabe Reef near Motiti Island.

Rena struck the Astrolabe Reef at 2.20am on Wednesday.

Maritime New Zealand's National Response Team was advised of the leak at about 10pm yesterday and plans were put in place overnight to prepare for responding at first light.

The national on-scene commander, Rob Service, says the flight this morning confirmed a slick of very thinly spread oil streaming from the vessel.

'The oil is very thin and it appears it is starting to break up and disperse naturally.”

Rob says the oil had leaked during pumping operations last night.

The National Response Team has set up a command centre at Bay of Plenty Regional Council and is assessing what the next steps will be.

Supplies from the national oil spill response equipment stockpile arrived in Tauranga overnight and have been readied for deployment this morning.

'While reports of a leak are unfortunate, they are not unexpected," says Rob.

'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.

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

The 236m cargo vessel struck the Astrolabe Reef near Tauranga Harbour at 2.20am yesterday. The Astrolabe Reef is about 4 nautical miles north of Motiti Island (about 12 nautical miles off the coast).

A Marine Pollution Response Service is working with council and environmental agencies and planning is underway to manage any impact on wildlife in the area.

Wildlife experts from Massey University went on an observation flight over the vessel yesterday to assess wildlife populations in the area.

They are working with local specialists, including the Department of Conservation to plan how to manage any affected wildlife.