The stern of the cargo ship Rena has settled on Astrolabe Reef, off the coast of Tauranga, after it began sinking earlier on Tuesday.
The ship has been grounded since 5 October last year and about three-quarters of the 150-metre-long section is submerged.
The ship's stern and bow broke apart after severe weather at the weekend, and the back section started slipping from the reef about 9am on Tuesday.
An attempt was made using the tug boat Go Canopus to reposition the stern on the reef, but Maritime New Zealand says that proved impossible.
Salvage master Paul van't Hof from the company Svitzer says no further movement of the back section has been seen since 10am on Tuesday.
"We believe at the moment (the Rena) is settled, although we cannot guarantee that she is 100 percent stable. Further assessments have to prove that ... but the situation is unchanged."
The bow of the ship remains on the reef.
Authorities are waiting for the weather to improve before the ship's condition can be assessed.
Oil and containers spill
Environmental clean-up company Braemar Howells says there are confirmed reports that 49 containers have spilled from ship since it broke apart.
Of those, 25 have come ashore by Tuesday. They are known to contain timber, paper, plastics and milk powder and none have hazardous goods.
A container has been seen as far north as Whiritoa near Whangamata on the Coromandel Peninsula.
Braemar Howells said two containers were dislodged from the Rena's stern on Tuesday morning. One sank and the other floated. It has since been towed by a tug to Motiti Island. There are 11 other pieces of containers adrift.
Eleven mini co-ordination units have been set up along the coast to work with local Maori and contractors to manage debris from the top of Coromandel to the Gisborne area.
Work is under way on Waihi Beach, from Papamoa to Kaituna Cut and on Matakana Island to clear debris, a Maritime New Zealand statement said.
Maritime New Zealand says its oil spill response team has placed the first boom at Maketu in anticipation of oil from the Rena reaching the shore on Wednesday.
The authority said it expected that freshly leaked oil from the ship would arrive at Motiti Island about 6pm on Tuesday and between Maketu and Matata from Wednesday afternoon.
Maritime New Zealand said the amount of oil released had not yet been calculated, but is known to be only a fraction of the size of spill released last October.
The oiled wildlife centre in Tauranga has been reactivated.
The Bay of Plenty Regional Council's Harbourmaster has directed all vessels to stay at least 200 metres clear of all containers from the Rena, as well as any salvage vessel working on the response.
These temporary exclusion zones are in addition to the existing exclusion zone, which is an area of sea within three nautical miles of the Astrolabe Reef and any part of the cargo ship.
The Port of Tauranga said modelling indicates any lost containers would drift southeast away from the reef. Normal shipping movements would continue, but extreme caution was needed by crews of ships coming and going from the port.