A large container ship ran aground on a reef off New Zealand's North Island Wednesday, releasing a "light sheen of oil" but not posing a major environmental hazard, the country's maritime authority said.

The 47,000 tonne container vessel "Rena" hit a reef about 12 nautical miles (22 kilometres) off the coast of Tauranga early in the morning, Maritime New Zealand said.

It said none of the 25-man crew of the Liberian-flagged ship was injured.

The agency said monitoring flights over the stricken vessel early Wednesday found some light engine oil had leaked into the sea but tanks carrying 1,700 tonnes of heavy fuel oil had not ruptured.

"While a light sheen of oil has been identified around the vessel, it has been assessed as hydraulic oil from the engine and does not pose a significant environmental risk at this point," it said.

"As far as we can ascertain, the ship is very firmly hard aground and is currently stable."

Incident controller Renny van der Velde said an oil spill response team was on standby to form a "safety net" around the vessel if the fluid began to leak.

"While the aim is to remove the vessel with no oil spilled, it is important to have contingency plans in place in case something goes wrong," he said.

Van der Velde said the reason why the ship hit the reef was still unknown.