As salvors and emergency responders prepare to pump oil from Rena, the 236m container ship grounded on the Astrolabe Reef, forecast bad weather has moved forward a day.

The New Zealand MetService is forecasting 30 knot northerly winds on Monday and these are likely to cause rough seas, potentially stalling any emergency effort.

See video below of salvage advisor Jon Walker's statement at a press conference in Tauranga this afternoon.

Fuel is continuing to leak from the vessel despite earlier reports today from Maritime New Zealand saying it had stopped.

On-water recovery operations manager Mick Courtnell estimates there is about 10 tonnes of oil in the oil slick.

There is about 1700 tonnes of fuel oil onboard the ship and it is the primary goal of the emergency responders and ship salvors to pump this off and transport it away.

Salvage advisor to Maritime New Zealand, Jon Walker, expects the pumping operation to begin late Sunday.

He says 20-40 tonnes of fuel can be pumped on to the Awanuia barge at this time.

The HMNZS Endeavour is due to replace the Awanuia on Monday and this will increase the efficiency of the operation as it can store a greater quantity of oil.

The speed at which the fuel can be pumped off Rena and onto the Endeavour is not known as it is dependent on many variables, including sea conditions.

Jon says their task is made difficult by a unique set of circumstances that make this operation one of the hardest he has handled in his 16 years of salvage experience.

He is confident the responders' effort to-date, and looking forward, is as good as it can be.

'You have got the best salvage team you could get internationally and they are using their resources from all around the world.”

Among the resources being brought to the ship is a Bell 214 heavy lifting helicopter.

'The vessel has no cranes so the equipment has to be flown out by helicopter.”

Use of a helicopter is complex as Rena's deck is stacked high with containers.

The ship is also on an 11 degree list and so moving equipment around on it is difficult.

The ship's position on the reef is a treacherous one as half of it is out of the water and Jon says salvors are assessing the impact wind and water are having on this precarious position.

'We are measuring motion of the vessel to learn how it is affected by the sea; if it's bending or flexing.”

This is secondary to combating the environmental danger, but is intertwined with that effort because if the ship bends and is damaged more that could increase the rate of oil leak.

The oil has claimed the lives of four seabirds and a further five little blue penguins and two shags have been found oiled.

They are in the care of the Wildlife Response Centre in Te Maunga.