Dangerous items were understood to be aboard the container ship - this morning that was confirmed as an alloy used in making steel.
The National Response Team was advised of the leak during pumping operations around 10pm last night.
The National On-Scene Commander, Rob Service, said the flight this morning had confirmed a slick of thinly spread oil streaming around 2000 metres in a narrow strip from the ship.
Mr Service said the on-water assessment would provide a clearer picture of what the oil spill response team was dealing with.
If the dispersant tests by experts were successful, it is likely a dispersant operation would be launched this afternoon.
The operation will only work if there's a significant amount of oil in the water.
Dispersant works by diluting the oil in the water and assisting its natural breakdown.
Mr Service said dispersant operations were only undertaken after careful consideration of the impact on the environment.
Representatives of the salvage company were now on the vessel and had confirmed the leak had stopped and only a small volume of oil had gone into the water.
An oil sheen in the water yesterday morning was put down to a hydraulic leak that was expected to disperse easily and not pose environmental risks.
Ready for the worst
Several specialist groups, including the maritime incident response team, wildlife experts and oil spill specialists the national response team have responded to the incident.
About 25 response team members have set up an incident command centre in Tauranga and would lead the response and be ready to act in the event of a large spill.
Maritime New Zealand said 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. 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," Mr Service said.
"We are ready to launch whatever level of response the situation requires."
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.
Dangerous goods on the vessel
Maritime New Zealand last night confirmed some dangerous goods were aboard the 236m cargo vessel.
The Liberia-flagged vessel, which was last night on a 10-degree list but sitting stable on the reef, is carrying 1700 tonnes of heavy fuel oil and about 2100 containers of varied products.
The content of the cargo was not known, although a manifest had been provided for review, Maritime NZ spokesman James Sygrove said last night.
"We understand, however, that there are some dangerous goods being carried - we are working to establish exactly what they are and how any risks can be managed."
Maritime NZ confirmed to ONE News this morning there is ferrosilicon on board, an alloy of iron and silicon used in making cast iron and steel.
A popular spot for fishing and scuba diving, with drop-offs of nearly 40m, Astrolabe Reef is home to an abundance of marine and bird life including petrels, little blue penguins and a seal colony.
However, none was considered to be affected, Maritime New Zealand said.
Salvage - to be undertaken by the vessel's owners - was expected to take "some time" and salvage advisers were due in Mt Maunganui this morning.
Last night, the vessel's 23 crew remained on board as pumps extracted water from two flooded cargo holds.
Investigations into why the ship struck reef
It remains unclear how the vessel hit the reef, and two separate investigations are under way.
Two investigators from the Transport Accident Investigation Commission are due to board the Rena this morning as part of an inquiry into the grounding.
The commission's inquiry, which was launched yesterday, will seek to explain how the grounding occurred and what, if anything, could be done to prevent a similar event happening again. It may also include the final extent of any environmental damage and aspects of the post-accident response and salvage.
The inquiry could take up to a year to complete, and the final report will be sent to the International Maritime Organisation in fulfillment of New Zealand's international treaty obligations for maritime accident investigation.
Tauranga sailor, Darryl Herbert, told Fairfax Media he could not believe the vessel managed to collide with the reef.
If you tried to run into the Astrolabe, you would probably turn 10 times and keep missing it," he said.
"It would be hard to hit even if you were trying."
Boaties were being asked to keep well away from the vessel - and even from the 1km exclusion zone established around it - amid concerns over safety and disruption to response efforts.
Before the zone was declared late morning, some sightseers in small boats were seen taking photographs and coming within metres of both the vessel and exposed parts of the reef.
"It's unsafe and they could have been endangering themselves," said Bay of Plenty regional harbourmaster Carl Magazinovic.
The vessel's listed owner is Liberia-based Daina Shipping and it is under the charter of Mediterranean Shipping Company.
In August, the vessel was detained for a day in Fremantle, Western Australia, by the Australian Maritime Safety Authority after "serious deficiencies" were found.
The authority's report found the vessel had "not been maintained between surveys", the "hatchway cover securing arrangements defective" and cargo was not stowed and secured as stipulated in the cargo securing manual.
But Port of Tauranga chief executive Mark Cairns said the port had had no previous problems with the ship.
