Today's release of formerly oiled little blue penguins is on hold as oil threatens to wash ashore on Western Bay of Plenty beaches.

Rough weather and strong swells during the weekend resulted in half a tonne of weathered oil leaking from the ship grounded on the Astrolabe Reef since October 5.

An observation flight on Sunday shows oil in the water around the Rena on Astrolabe Reef.

Maritime New Zealand spokesperson Sophie Hazelhurst says this may result in light oiling and 'sticky tar balls” washing ashore.

'There may be a few tar balls washing up on beaches, but this is very minimal compared to what we have seen.”

Sophie says the planned release of penguins on Mount Maunganui Main Beach today is postponed due to the threat of further oil leaks.

'There was potential for more oil to come out in the weekend, and with bad weather forecast last night we wanted to be careful.”

She says the release will still go ahead as soon as the habitat is deemed clean enough and the risk is minimised.

The Rena was carrying 1700 tonnes of heavy fuel oil when it ran aground, with about 350 tonnes of heavy fuel oil spilling into the ocean during a storm on October 12.

More than 340 little blue penguins were found oiled and treated at the wildlife centre in Te Maunga.

The first 49 penguins were released back into their habitat on November 22.

Rena suffered extensive damage to its hull when it grounded on the reef and oil has intermittently leaked from its duct keel.

This is a system of pipes running along the bottom of the ship, since the grounding.

The oil was first spotted by salvors working onboard the Rena on Saturday night and confirmed in an observation flight on Sunday.

Darker patches of oil can be seen floating to the side of the wreck.

MNZ shoreline clean-up assessment teams are checking the beaches today and clean-up crews are working at Mount Maunganui, Papamoa and Matakana Island.

Efforts to lift the remaining 1114 containers onboard the Rena remain on hold due to bad weather.

There are northeasterly winds of 15 knots shifting to northwest today with a maximum swell height of 4m.

Container removal operations are not expected to commence for another two days with a further weather spike expected on Tuesday night.

Electronic sensors monitoring the movement of the wreck indicate some extra movement on the reef on Sunday night due to large sea swells, but the Svitzer salvage team advises there is nothing 'untoward” in the readings.

Salvage teams are boarding the wreck this morning to resume work on installing patches and passages to improve the buoyancy of the ship.

The barge vessel, Sea Tow 60, which lifted containers from the Rena, is back in port, while the larger crane vessel Smit Borneo is expected to arrive today from Singapore.