Authorities are hoping an oil dispersant may be more effective than initially thought, as efforts to salvage a grounded cargo ship off Tauranga continue.

Fears are growing the stranded ship Rena, which is haemorrhaging oil into the sea, has the potential to cause a major environmental disaster.

The 236m cargo vessel struck Astrolabe Reef near Tauranga Harbour early on Wednesday, causing a 5km oil slick that has already killed some wildlife.

Last night, authorities admitted that a dispersant they were using on the 5km oil slick leaking from the MV Rena was not working.

Oil spill response teams and wildlife workers were today travelling to the area and experts are now looking to use specialist equipment to scoop the oil out of the water.

Maritime New Zealand national on scene commander Rob Service said a flight over the ship this morning found a sheen of oil is still spreading from the ship but there is no evidence of further spillage of heavy fuel oil. Observers on the flight said there was no sign that oil is moving towards Mayor Island at this stage.