Rena has finally begun to sink and experts believe it's only a matter of time before the front part of the ship sinks below the surface.
Experts noticed the on-board motion sensors significantly changing around 9am and less than an hour later, the rear half of the ship had started to sink.
About 75 per cent of the stern section of the ship is today under water.
About an hour after the motion sensors began noting significant changes, it was almost completely under water.
Maritime New Zealand said the stern section of the ship rapidly slid off the reef and sank in a very short time.
The bow of the ship then settled on the reef despite being almost completely submerged.
Yesterday, the accommodation part of the ship, or the bridge, had almost entirely vanished below the waves and only a tiny section of the bridge (on the port side) poked above the crashing waves.
Salvors face one of the most dangerous and expensive underwater operations ever undertaken in New Zealand as they try to recover hundreds of underwater containers, says a salvage expert.
New Zealand Diving and Salvage general manager Howard Saunders says a technique known as saturation diving is likely to be used after the ship's stern began its descent yesterday.
Mr Saunders said that by law the Rena would have to be removed after it sank the 80 to 90 metres to the seabed.
That would require one of the biggest and most dangerous salvage operations undertaken in New Zealand.
He said the salvors would probably be working in depths of greater than 30m, so they would have to use a saturation diving spread - rarely seen in New Zealand - with pressurised chambers and diving bells that enable divers to live and work safely at depth for days and even weeks.
One is in operation off the Taranaki coast at a natural gas rig.
"You can live down there with food ... sometimes they call them habitats and people live on the bottom, but most modern operations are not that long."
He said the underwater salvage job would be among the biggest projects in New Zealand's maritime history and possibly among the most expensive.
Some saturation dive teams charged US$150,000 dollars (nearly $190,000) a day.
Maritime New Zealand confirmed about 400 containers were still in the stern section of the ship, which primarily was below the surface.
Two containers popped out of the stern section as it sank.
Maritime New Zealand said one container sank and the other floated and was taken ashore by a tug.
A number of life rafts also inflated.
Maritime experts are today advising locals to brace themselves for an influx of oil to wash up along the coastline.
Alex van Wijngaarden, Maritime New Zealand National On Scene Commander, said the trajectory modelling of the oil showed it was likely to reach the beach between Maketu and Matata late today.
As well as oil, debris and mangled pieces of iron, which were once containers, are expected to wash up from Whitianga to Paengaroa sometime today.
A small amount of oil had been released from the ship since it began to sink yesterday morning.
Environment Minister Nick Smith said the amount of oil released was "in the single digits of tonnes".
Meanwhile, oil spill response teams were preparing to clean up the oil, which included placing booms in sensitive areas along the coastline, Mr van Wijngaarden said.
He said salvors had equipment at the reef to try to contain any oil escaping from the wreck.
The wildlife centre in Tauranga would be reactivated if needed. The volunteer Adopt-A-Beach programme has been reactivated.
Meanwhile, bad weather has forced container recovery to come to a temporary halt.
Maritime New Zealand said divers would assess the condition of the hull before container removal would begin again.
Svitzer Salvage Master Paul van't Hof said until the swells subside, it was not possible to assess the condition of the submerged stern section or carry out underwater surveys.
This meant the salvage team could not plan any further salvage operations.
Sensors on the bow section of the Rena showed it had been stable since 10am yesterday, he said.
Salvors have identified 11 pieces of containers adrift.
Claudene Sharp of Braemar Howells, the company responsible for the processing of containers, said they had deployed 13 vessels, ranging from tugs and barges to a fast response vessel, from the Waihi Beach area south to Motiti.
"We have identified 49 containers since the Rena split at the weekend and of those, around 25 are beached," Ms Sharp said.
The beached containers contain timber, paper, plastics and milk powder. None contained hazardous goods.
Thirteen containers have washed ashore at Waihi Beach, of which four have already been removed. One was destroyed in rough seas and eight remain in the area.
The furthest north that containers have been found is Whiritoa, north of Waihi Beach.
Today there were oil spill and clean-up teams on Matakana Island and on the coast at the Mount and Maketu with booms to prevent oil washing ashore.
6.15am: Maritime New Zealand salvage crew flew over Rena and observed a slight change in the vessel.
7.45am: Svitzer salvage crew noticed on-board monitoring systems had slightly changed.
8.30am: The motion sensors on board the vessel showed a significant change. At the same time, the anchor tug Go Canopus began an attempt to reposition the stern section on the reef to make it easier for the container recovery barge Smit Borneo to move closer to the Rena once container operations resumed. This proved impossible due to the weight of the ship and the water entering the vessel.
9.20am: A helicopter with salvors landed on the bow section of the Rena to monitor the condition of the stern, which progressively sank between 9.30am and 10am.
10am: The stern section has been stable since 10am, with about 75 per cent underwater including the bridge.
- Maritime New Zealand