The stern section of the Rena began slipping from its previous position on the Astrolabe Reef in the morning and was about three-quarters submerged by noon New Zealand time, Maritime New Zealand spokesman James Sygrove told The Associated Press.
"The front 30 meters (100 feet) is still above the waterline, but the back section and the bridge are all under the water," he said.
Sygrove said the bow section of the boat remained firmly wedged on the reef. He said there is plenty of wood, plastic and other debris floating around the sinking stern section.
"It's quite a fluid situation," he said, adding that authorities remain unsure of what will happen next.
The mostly submerged stern still was perched on the edge of the reef and a small amount of oil and some containers fell overboard along with the debris, the maritime agency said in updates in the afternoon and evening. Cleanup teams were prepared if oil or any items washed ashore on the New Zealand coast.
The 774-foot (236-meter) vessel has been battered by heavy seas since it ran aground Oct. 5 and the ship broke in two over the weekend. Astrolabe Reef is 14 miles (22 kilometers) from Tauranga Harbor on North Island.
About 150 cargo containers have spilled into the sea since the weekend, with more than 800 still aboard. Many more containers are expected to fall off as the stern sinks.
In the days after it ran aground, the Rena spilled about 400 tons of fuel oil, fouling pristine beaches and killing thousands of seabirds in what has been labeled New Zealand's worst maritime environmental disaster.
Maritime New Zealand estimates that less than 100 tons of oil remains on the ship after salvage crews managed to remove much of the remaining oil and nearly 400 containers. However, it was a slow process removing containers and hundreds were still aboard when it split apart.
New Zealand police closed access to one beach Monday after some people were seen scavenging bags of powdered milk that had washed ashore. Authorities warned that the milk may be unsafe.
More than 30 containers and plenty of loose debris has washed up at local beaches since the Rena split apart, and authorities have been working to tow other containers out to sea to prevent more from coming ashore. Salvage crews have attached beacons and buoys to some containers so they can be more easily recovered later.