Locating devices called "pingers" have been fastened to cargoes aboard the ill-fated MV Rena as the salvage team moves to finish pumping oil off the grounded cargo vessel.

It has been more than a month since the Rena accidentally hit a reef and got wedged in, crippling the vessel, and causing oil spills around the Bay of Plenty - the worst maritime disaster in New Zealand.

Salvagers working aboard the boat have fixed 35 pingers and 200 more will be attached to the rest of the containers holding dangerous goods and those likely to fall off the ship.

Salvage master Captain Drew Shannon said his team will get to as many cargoes as possible.

Meanwhile, the remaining 358 tonnes of oil from Rena will require about 100 hours of continuous pumping in the right conditions.

"There is still a strong likelihood further oil will be released, but the more oil removed from the ship, the lower the risk," Maritime New Zealand national on-scene commander Rob Service said.

As lifting of cargoes got under way, a 6000-tonne container barge was successfully tested for the next stage of operations. The ST60, chartered from Australia, carries one 280-tonne crane to free the cargoes from their locks and transfer them.

"There have been other container ships placed in similar situations and we are drawing on industry to assist with the method and means of actually removing the containers," said Shannon.

Shannon added every ship has a different procedure for locking, so his team will continue to study the situation as they go along.

The Rena's captain and second officer are facing charges under section 65 of the Maritime Transport Act which relates to operating a vessel causing unnecessary danger or risk to a person or property. The charge carries a maximum penalty of $10,000, or a maximum term of imprisonment of 12 months.