Air pockets that have delayed the pumping of heavy fuel oil from the container ship Rena on Astrolabe Reef for 24 hours appear to be resolved.

Maritime New Zealand salvage unit manager Arthur Jobard says salvors resumed pumping heavy fuel oil from the starboard five tank at 6pm on Wednesday.

This graphic shows inside Rena. The starboard five tank is shown with the bottom half full of the water that was pumped in to raise the oil to the top of the tank for hot tapping. Graphic: Rosalind Spink, LOC.

The pumping continued through the night and by 8am today, 31 tonnes of an estimated 358 tonnes, had been transferred to the tanker Awanuia.

It may be the end of a frustrating process for the salvors who have reset their pumps several times only to find them sucking air.

The process of clearing and resetting the pumping system takes several hours.

The air pockets are thought to have been located in the ship's structural framing within the tank.