Filipino sailors from the ship responsible for New Zealand's worst sea pollution disaster are being kept at a secret location amid fears for their safety, the ship's agent said Friday.

As salvage crews raced to stop more oil seeping from the stricken Rena, which hit a reef last week, Mediterranean Shipping Co. (MCS) said the six sailors still in the country were staying at an undisclosed location.

The Philippine embassy said 19 of the 25-man crew had already flown home after being interviewed by New Zealand authorities over the disaster in the ecologically sensitive Bay of Plenty, which teems with wildlife.

Two of the remaining six -- the captain and second officer -- were charged this week over the October 5 accident. A Tauranga court ordered their identities be suppressed after lawyers raised concerns for their safety.

MSC, which had chartered the Rena, said there had been no direct threat to the sailors but that they were being kept in a secret location as a precaution.

"We're keeping them low key, at an undisclosed location," said shipping agent Mike Hodgins.

"We're not going to parade them in front of everyone in case some nutcase does something he shouldn't," he told AFP.

Hodgins said they were not under police protection and most people in the North Island community were taking a common-sense approach to the disaster that has blackened their beaches, although he had seen some anger toward the crew.

"I've been out in town and heard some people saying things, but it's just talk," he said.

Filipino community group Migrante Aotearoa New Zealand issued a statement headed "Filipinos appeal against racial hatred over Rena disaster" after one of its members, a parking attendant, was abused in an Auckland street.

"We don't want this to become a racial issue, Filipinos are as upset as anyone about this disaster... I hope this is an isolated incident," the group's coordinator Dennis Maga told AFP.

He said the Filipino community planned to organise a volunteer drive to help in the clean-up of affected beaches.

Tauranga mayor Stuart Crosby said frustrations over the accident should not be vented on the crew.

"In some respects that crew are victims as well, and I'd like to think that we'd be looking after them in a proper manner, as Kiwis should," he told TVNZ.

Maritime New Zealand (MNZ) said the slick, which has dumped clumps of oil on beaches, had thinned but was becoming wider. Whakatane, 90 kilometres (56 miles) east of Tauranga, is bracing to be hit.

A massive clean-up operation is under way along the coastline involving 1,000 people, MNZ said.

The government agency said 1,000 dead birds had been recovered and teams were trying to round up seals in the area. They had caught three so far.

Maritime salvage crews resumed a "highly risky" operation to try to pump remaining oil from the Rena, which is teetering precariously on the reef and threatening to spilt as huge cracks gape in its hull.

"The moment there is a sense that it may be too dangerous, or if anything changes, they will be getting back off that vessel quick smart," Matt Watson, whose company Svitzer is leading the salvage effort, told Radio New Zealand.

Up to 700 tonnes of oil has leaked from the vessel and salvage crews hope to drain the Rena's fuel tanks to stop all 1,700 tonnes it was carrying from spewing into the sea.