Almost six months to the day after the Rena struck Astrolabe Reef on October 5, the former container ship's stern section has slid down the reef and sunk.

The stern section was found to have submerged during a flight over the reef, about 25km from Tauranga, at 4pm on Wednesday.

Only the bow section of Rena remains above water at Astrolabe Reef. Photo: MNZ.

An earlier flight found the ongoing pounding from rough seas, with waves of up 12metres recorded on Tuesday night, rocked the stern section beginning the sinking process.

Earlier reports put the stern section on the reef slope, which falls away to the sea floor at about 80 metres depth.

A light sheen of oil has been seen stretching for about 1km north-west of the wreck.

Maritime New Zealand advises the sinking of the aft section will likely result in the loss of more debris and small amounts of oil into the sea.

However, most of the oil has been removed from the wreck and MNZ says any oil released by storm surges will be a small percentage of the hundreds of tonnes of oil that was released during a storm in October, which washed ashore creating one of New Zealand's worst environmental disasters.

On January 8 the 236m container ship split in two, spilling up to 300 containers into the sea.

The separated stern section slid down the slope of the reef a few days later, submerging the accommodation section and rest of the aft section, except for the port bridge wing.

An image of Rena on the reef after it broke in two in January.

The forward section of the vessel is still on the reef, but has suffered further damage to its rear section.

Winds of 27 knots (50km/h) and seas of 8-9m are expected at Astrolabe Reef today.

MNZ's oil spill response team and specialist container recovery teams and vessels from Braemar Howells remain on alert to respond to any reports of further oil or debris coming ashore.

Strong winds and rough seas continue to impact on-water debris recovery operations, which will resume as soon as conditions allow.

MNZ and the Svitzer salvors will continue to monitor and assess the condition of the wreck, and will attempt to undertake a closer inspection of the wreck when conditions ease.

Boat skippers and vessel masters are reminded to take extreme care, as the debris from the wreck poses a significant risk to shipping. A 2nautical mile exclusion zone remains in place for the Astrolabe Reef, and navigational warnings continue to remain in force.

Click here to see full coverage of the Rena.