UPDATE: Four vessels have been working since early this afternoon to corral an estimated 5 to 10 tonnes of oil spilled overnight from the cargo vessel Rena.

National On Scene Commander Alex van Wijngaarden says it was probable the oil had escaped from Rena's duct keel. The duct keel is the tunnel running the length of the vessel that houses pipework and other service equipment.

Images of the initial oil spill from Rena on October 6.

Despite opening the beach to swimmers this morning, 10 tonnes of oil has leaked from the Rena overnight, according to Maritime New Zealand (MNZ).

In a statement, MNZ says it is unclear exactly where on the ship this has come from, but it is considered probable the seepage is a result of further leaking from the duct keel due to the low tide.

MNZ says four vessels were being prepared to go to the site as part of an on-water recovery operation. Heavy-duty booms would be used to "corral" the oil and skimmers would then remove it from the water.

A Defence Force helicopter would also be launched to direct the ships to the oil flow.

MNZ says planning teams are now doing trajectory modelling to assess where the oil is likely to head.

"At this point it is moving offshore, and it is not expected to reach any coastline for the next two days."