Nearly 8,000 people have volunteered to help with the clean-up operations in the Bay of Plenty on the north part of New Zealand, where a cargo ship was grounded on a reef more than four weeks ago.

Among the volunteers, 25 have lived on Motiti Island for the past five days, near the wreck of the "Rena" on Astrolabe Reef, as they remove oil and debris from its rocky shores, according to Maritime New Zealand (MNZ) on Friday.

Local residents have been regularly working on the island since Rena grounded. Oil from the ship's fuel tanks had washed up on the northern shores of the island.

Operation Beach Clean manager Pim De Monchy said the call went out last week for registered volunteers who were prepared to spend a week on the island.

The people who responded to the call provided enough cover for teams to be rotated if necessary over the next couple of weeks.

They were also joined by Australian rock clean-up specialist Peter Braddock, who provided advice on cleaning oil from the rock pools.

"We are fortunate in a sense that Motiti Island has a coastline with rough seas. This actually helps with the removal or degradation of the oil," De Monchy said.

A total of 7,773 people had now registered with the volunteer clean-up program, according to MNZ. All volunteers have been equipped with protective clothing suitable for the environment they are working in.

Meanwhile salvers have been trying to speed up the removal of the fuels left on the cargo ship.

MNZ salvage unit manager Bruce Anderson said a team of 22 salvers were on board the Rena on Thursday, pumping the lube and hydraulic oils in the engine room into a centralized tank. Three of them remained on Rena overnight to keep the system going.

He was hoping to begin pumping these fuels onto another ship Awanuia by the end of Friday.

Another team of divers continued to work on re-establishing the dive station and creating a dam so oil could be pumped from the submerged starboard tank. The starboard tank contains 358 tons of oil and is the most difficult tank to access, according to Anderson.

Bad weather earlier this week prevented salvers from working on board the Rena.

It was feared it might break up on Tuesday night, when five meter swells lashed the vessel. But it remains in one piece, and officials have warned the ship may be stuck on the reef for months or even years.