Rena's splitting and sinking of its stern is increasing marine pollution levels with more oil spewing into the sea and industrial substances washing ashore onto Bay of Plenty beaches.

The vessel's stern section began slipping off the Astrolabe Reef at about 8.30am and by 11.30am it was almost entirely underwater.

The ship's bow remains fast to the reef, close to where it originally ran aground on October 5.

Large amounts of oil can be seen emanating from the wreck although Maritime New Zealand says the quantity is less than spilt into the sea last year.

It is not seeping from one point, but instead most of the water surrounding the broken vessel has taken on an oiled hue.

Oil is leaking in many directions from the Rena wreck at the Astrolabe Reef. Photo and video footage recorded by SunLive at about 11.15am during an aerial observation flight of the wreck.

Maritime New Zealand national on scene commander Alex van Wijngaarden says trajectory modelling shows the first of this newly released oil will wash ashore at Motiti Island this evening.

'This was not unexpected,” says Alex, 'we are prepared, and we will deal with it.”

More oil is predicted to reach Pukehina on Wednesday.

This oil spill follows a considerable rainbow sheen of oil on the sea's surface that today drifted into Tauranga Harbour through its northern entrance.

Places near the entrance; Waihi Beach and Matakana Island, are the most badly polluted by floating containers and debris to fall into the water following Saturday's storm.

It was this storm that broke the Rena into two pieces putting a 20-30m gap between them.

Rena's stern slips off the Astrolabe Reef. Photo SunLive (11.15am).

The sinking provides an opportunity to see the inside of the Rena. Photo: SunLive (11.15am).

Oil spill response teams are preparing to clean up the newly released oil, including placing booms in sensitive areas.

The oiled wildlife centre at Te Maunga is being reactivated.

The volunteer programme is also reactivated, with volunteers asked to register at www.boprc.nz/oilspillvolunteers

Debris spills from Rena's stern as it begins to slip off the reef. Photo: LOC

The debris spread quickly from the Rena. Photo: LOC.

More containers are expected to have come free from the vessel owing to this morning's sinking of the ship's stern section.

Container recovery company Braemar Howells estimates that 400 containers are in the stern section of the Rena.

Its spokesman, Grant Dyson, says two tugs have been sent to the Rena to try and contain drifting containers and also tow any floating containers to a specialised recovery barge.

Most of the debris sighted in the water and on beaches on Monday was timber, polystyrene and milk powder bags.

Some other items, such as wheelbarrow wheels and a sofa were found on Matakana Island.

Debris from the Rena wreck litters the ocean side beach of Matakana Island. Photo: SunLive.

On Matakana Island's ocean side beach today some new substances have been found.

These include 25kg bags of low density polyethylene (ldpe). This is a substance used in the manufacture of light plastics such as kitchen containers.

Speaking from Matakana Island, Sun Media director Brian Rogers says these granules are covering at least 1km of beach.

This is about 7-8km from Mount Maunganui.

A handful of low density polyethylene is but a sprinkling of the total amount of this chemical on Matakana Island.Photo: SunLive.

The bags the granules were transported in aboard the Rena appear similar to the milk powder bags seen at beaches in the northern Western Bay of Plenty on Monday.

Brian says other pollution on the beach includes ‘egg fried rice' microwave meals, thousands of pottles of rice, skirting board and plywood.

There is also oil washed up on the ocean side beach. It is up to the high tide mark and is mixed with seaweed and other sea life stranded there.

A Maritime New Zealand clean-up crew is working on the beach to clear this oil.

Oil granules and seaweed on Matakana Island. Photo: SunLive.

Oiled covered microwave meals litter Matakana Island today. Photo: SunLive.

A Maritime New Zealand beach clean-up crew at work on Matakana Island. Photo: SunLive.

Clean-up crews on Matakana Island dodge debris as they seek oil pollution. Photo: SunLive.