There is less than 1000 tonnes of heavy fuel oil aboard the container ship stranded on the Astrolabe Reef about 25km offshore from Tauranga.

Rena ran aground three weeks ago, at about 2.20am on Wednesday, October 5, and has since leaked an unknown quantity of oil into the sea and lost 88 containers into the water.

The Awanuia is giving way to the Go Canopus as the bunker barge receiving oil from the Rena.

Maritime New Zealand's focus is pumping the oil out of Rena's fuel tanks and into bunker barges stationed alongside.

Of the about 1700 tonnes of heavy fuel oil originally onboard, the salvage team has pumped about 740 tonnes off.

This was pumped from the port number five tank onto the Awanuia barge, a vessel being replaced in the pumping operation tonight by the Go Canopus.

MNZ salvage unit manager Bruce Anderson says this barge has a dynamic positioning system to help it function in rough seas where there is risk of collision.

GPS is used to trigger its engine so that it moves in tandem with Rena – maintaining a fixed distance between the vessels.

It is being positioned differently to the Awanuia, which is now stepping off duty after receiving 772 tonnes of oil out of port five tank.

There is just 35 tonnes of oil left in this tank and Go Canopus can store up to 150 tonnes of oil at a time.

The most difficult operation for the salvage team is extracting 358 tonnes of oil from starboard five tank and divers are preparing spaces near it to commence pumping.

This includes constructing a coffer dam underwater in dangerous conditions.

While sawing a pipe today, a diver found what he was cutting through was still live, showing electricity is still running through parts of the ship.

The power was quickly cut to this section of the ship.

The weather forecast is good for the next five days and the ship is stable on the reef, with Mr Anderson saying its stern and bow movement differential is being closely monitored.