The ill-fated MV Rena has further split in two pieces in a bout of bad weather over the weekend, and the salvors estimate that only 20 per cent of its 200 to 300 containers could be saved now that the cargoes have been washed overboard.

Heavy swells up to six metres pounded the Rena on Saturday night.

Weather forecasters had earlier warned that approaching bad weather could be catastrophic to Rena, which ran into Astrolabe Reef in New Zealand's Bay of Plenty more than three months ago. It caused a major oil spill and has been stuck in the reef since then, while salvors cleaned up and started planning operations to get the containers safely.

The ship's stern is now listing at 23 degrees to starboard while the bow section remains firmly wedged on Astrolabe Reef, the New Zealand Herald reported.

Maritime NZ officials flew over the vessel Sunday morning to assess the situation. Grey waters marked the location of the ship as tonnes of milk powder from one of its containers spilled out when it was washed overboard.

At a news conference, Claudine Sharpe, spokeswoman for environmental cleanup specialist Braemar Howells, said 30 containers have so far been identified, and 15 of those tagged and corralled in an offshore area. The vessel has "lost quite a lot of containers," she said.

As the containers had earlier been tagged with transponders, Sharpe was confident that containers that float would be recovered.

Sharpe estimated that Rena was carrying 200 to 300 containers, and 20 per cent of those are hoped to float, while the rest would inevitably sink.

Rena's salvage team is ready in the event that containers are washed ashore.

"We will deal with it but as I said our main priority is to stop it coming ashore if we can," she said.

Maritime New Zealand salvage unit manager Dave Billington said his team has been receiving reports of containers being lost since about 8 p.m. Saturday night.

Fears rise anew that the split could result in another oil spill as there were circulating reports that oil was spotted ashore near Rena's location.

However, officials said it was very unlikely that oil reported ashore Sunday was from the Rena's split, and it could be old oil that was moved ashore by the rough waters.

"Any oil coming ashore in the coming days is expected to be much less the amount that washed up after the Rena first went aground," said National on Scene Commander Alex van Wijngaarden.

Wijngaarden advised the public to keep away from the nearby beaches to be safe from the debris and sea conditions.

"We reiterate the message that people need to exercise their common sense and not swim or surf where there is likely to be containers, debris or oil coming ashore. Also we ask that people keep clear of any debris or oil, and report it to us so that our trained people can come and clean it up," he said.

Bay of Plenty Regional Council water management group manager Eddie Grogan said the regional council was currently reassessing the three-nautical mile exclusion zone around the Rena, the Herald reported.

"We will provide more information once we've assessed the situation, however we anticipate the exclusion zone will be increased,"' he said.