The oil leaking from the 236m container ship grounded on the Astrolabe Reef appears to be getting thicker and darker in colour.

The Liberian-flagged Rena ran aground at the reef at about 2.20am on Wednesday and has leaked an unknown amount of oil.

SunLive reporter Phillipa Yalden handles some of the leaked oil at the perimeter of the 1km exclusion zone from the Rena shipwreck at the Astrolabe Reef.

The last Maritime New Zealand estimate was that there was about 1500 tonnes of oil onboard the vessel.

The leaking substance is a fuel oil and for the last two days it has intermittently leaked into the sea – leaving a slick estimated to be 5km long, noticeable from the air by a sheen.

Reporting from the Waterline magazine boat, SunLive journalist Phillipa Yalden says the oil in the water about 1km from Rena appears to be denser than it was on Thursday.

'It's black, sticky and horrible,” says Phillipa.

'The slick is inconsistent with chunks of black blobs floating through the water.”

In a statement released by Maritime New Zealand at about midday, its national on scene commander, Rob Service, says a relatively small amount of heavy fuel oil has escaped.

The leak is from pipes on the ship, with Rob saying at a press conference on Thursday that there did not appear to be a breach to the ship's fuel tanks.

The emergency responders have undertaken aerial surveillance of the oil slick this morning and report it is still spreading from the ship, but there is no evidence of further spillage of heavy fuel oil.

They also say there is no sign that oil is heading towards the marine and birdlife reserve Mayor Island.

Four birds were found dead on the water at the slick near the ship on Thursday.

There is more activity on the water around Rena today, with vessels preparing for an on-water response to collect oil on the water.

Rob says current assessments are that the on-water responses may be of limited value at this stage because of the way the ship is situated and the spread of the oil.

Rena today, with a divers' tender alongside, supporting divers who are examining the damage to the ship.

These efforts are coming after earlier trials of the dispersant Corexit 9500 did not appear to have been successful in dissipating the oil slick.

Helicopters several kilometres to the north of the shipwreck are operating and these are believed to be conducting trials of dispersants on the slick there.

Teams of wildlife rescue staff have been sent to Motiti Island and Maritime New Zealand is also preparing to launch a shoreline clean-up in the event that oil reaches land.

Over 90 people are involved in the response, with more being brought in as the wildlife and shoreline teams are expanded. International expertise is also being called on.

A wildlife rehabilitation centre to treat oiled birds is established at the Te Maunga Wastewater Treatment Plant.