Officials are "very, very worried" about a stranded cargo ship spilling oil into the sea off the coast of Tauranga.
It's not yet known where it's coming from, but engineers are on board the Liberian-flagged vessel, Rena, checking for the source.
The 236-metre ship had its hull damaged when it hit Astrolabe Reef early on Wednesday morning and remains stuck fast.
An aerial survey on Friday will determine if the 5 kilometre-long oil slick has grown or changed direction.
A toxic oil dispersant has been sprayed in a bid to break the oil up, but the manager of Maritime New Zealand's (MNZ) pollution response service, Andrew Berry, says it didn't work and they will up the amount of dispersant used or look at other chemicals.
"Clearly we'd prefer not to have to spray it, (but) compared to the greater evil of large quantities of heavy fuel oil it's, in our opinion, it's a considered risk worth taking," Mr Berry told Radio New Zealand.
MNZ will meet with the salvage company, Svitzer, to come up with a plan to get the 1700 tonnes of fuel off the ship before the weather deteriorates over the weekend.
The reef - about 4 nautical miles north of Motiti Island and 12 nautical miles off the coast - is home to animals including little blue penguins, seals and petrels and four oiled birds have already been found dead near the ship.
Earlier this year, the Australian Maritime Safety Authority detained the Rena for more than a day after it was found to have a number of deficiencies including a faulty hatchway, cargo not stowed or secured, and cracked and rusted parts.
Mr Berry says the latest incident doesn't seem to be linked to these problems, but investigations down the track will shed more light.
NZN