The captain of a container ship wedged on a reef off New Zealand has appeared in court over its grounding as more thick fuel-oil fouls beaches in the country's worst environmental disaster in decades.

The 47,230-tonne Liberian-flagged Rena has been stranded on a reef 12 nautical miles off Tauranga on the east coast of New Zealand's North Island since running aground a week ago.

The captain has been charged "for operating a vessel in a manner causing unnecessary danger or risk", which carries a maximum fine of NZ$10,000 ($7,810) or 12 months in prison.

Local media reported the ship's master, a 44-year-old Philippine national, made a brief appearance in court and was bailed for one week without making a plea. His name was suppressed.

Heavy swells and strong winds pounded the vessel for a second day on Wednesday, sending empty containers tumbling off the ship, which is leaning at around 18 degrees, into the heaving seas.

"The bow appears to be still firmly wedged onto the reef, we have lost more than 30 containers from the stern and midships," Maritime NZ director Catherine Taylor told reporters.

Some of the containers were reported to be bobbing in the heavy seas with others washed up on a small island, Motiti, about five miles from the ship.

The ship was carrying 1,368 containers, 11 of which are said to have hazardous substances in them. Shipping using the port of Tauranga, which is the country's biggest export port, was being rerouted away from the containers.

Authorities said the bad weather was helping to break up and disperse the estimated 300 tonnes of oil that escaped from the ship.

"That's a little bit frustrating because once the oil is on the beach we can actually deal with it, we can remove it from the beach relatively easily," said Ian Niblock, a spokesman for the clean-up operation.

Oil is scattered along 16 miles of the district's long, golden beaches, which are a magnet for surfers. Nearby waters have an international reputation for big-game fishing.

Several hundred people were scraping clumps of thick, toxic fuel oil, some as large as dinner plates, into plastic bags and large bins.

Booms have been placed over some harbour entrances to keep oil out of wetland and wildlife habitats. About 50 seabirds have died and teams of naturalists have scrubbed and treated 20 more for oil contamination.

Refloating and salvage of the ship are the responsibility of the owner, Daina Shipping, a unit of Greece's Costamare Inc, as well as salvage experts, but any plan needs official approval.

A floating crane able to remove containers from the ship is on its way from Singapore.