New Zealand Maritime has said that salvage crews are facing difficulties as they try to remove containers from the grounded Rena cargo vessel.

On Monday officials said all the oil had been pumped out of the ship.

But subsequent plans to use workers in a cage suspended from a crane to remove the 1,300 containers were proving "too dangerous", the organisation said.

The Rena, which got stuck on a reef last month and was in danger of breaking apart, initially leaked 350 tonnes of oil, killing more than 1,000 sea birds.