A dive team confirmed Sunday that the submerged part of the stricken MV Rena is intact, reassuring salvage officials that oil recovery would go on without further hassle as long as weather stays favourable off Tauranga in New Zealand.

An estimated 5-10 tonnes more oil leaked from the cargo ship on Saturday, but the salvage teams said the leaking did not last overnight.

"The fact that divers found the starboard tank intact is excellent. We hope to start recovering oil from this tank soon," Maritime NZ salvage unit manager Bruce Anderson told the New Zealand Herald.

The good weather in the area had allowed the Rena's salvage operations to move forward and make progress. Weather forecasters said conditions will be favourable until at least Thursday, and further plans will be made before then to guide the salvage teams.

Meanwhile, the fuel transfer team on board the Rena is now using a better system with doubled pumping capacity to maximize oil recovery efforts. The new hose has a 15cm diameter, which is twice the size of the previous hose.

"That hose is used to take fuel out of the tank, and it then splits into two separate three inch (7.5cm) hoses, each fitted with booster pumps. Those hoses then take the fuel to the [tanker] Awanuia,'' he said.

In Mount Maunganui beach, swimmers have been allowed entrance again on Sunday, but the swimming area remains limited in the open section of the beach between the mountain and Tay Street.

MNZ National On Scene Commander Alex van Wijngaarden told NZ Herald that public health officials had confirmed it was safe to swim, as long as people continued to stay within safe areas. The rest of the beach, from Tay Street to Maketu, remained closed to the public.

"We've now had 12 days without any significant release of oil from Rena," he said, adding, "People should be careful and look out for any sign of oil contamination. Although the residual oil is a lot less toxic than fresh oil, people should still avoid it."

As this developed, endangered orca whales have been spotted coming from the area where oil has spilled from Rena, raising fears that the whales may have ingested the toxic oil.

"There was one fairly large male and two females. They would have been about 30m off the shore. They were heading towards Whakatane," Bay of Plenty resident Paul Carter told NZ Herald.

Orca biologist Dr Ingrid Visser said it was likely that the whales had ingested oil, and this could cause liver damage and other major illness to the mammals.

Vesser further said there were less than 200 living around the New Zealand coastline, and they are considered to be under the 'most endangered' category among animals.

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