Flights between Australia and New Zealand were returning to normal Friday after an ash cloud spewed by a Chilean volcano moved away, almost a week after causing trans-Tasman havoc.

Australia's Qantas, which had banned all flights to New Zealand since Sunday, said it had resumed services to and from Auckland, Wellington and Queenstown but Christchurch routes remained suspended until 0600 GMT.

"Qantas will continue to monitor the movement of the ash cloud and assess its impact on flight operations," the airline, which resumed all domestic services on Thursday, said in a statement.

Qantas offshoot Jetstar has also restarted New Zealand flights, along with Virgin Australia as the cloud moves away from New Zealand airspace.

Air New Zealand, which had operated some trans-Tasman services by flying under and around the plumes, said Friday it was now operating to all of its domestic and international ports as normal.

Volcanic ash from Chile's Mount Puyehue volcano reached airspace over Australia and New Zealand last Saturday, shutting down flights to Melbourne, Adelaide and Perth during the week and stranding thousands of travellers.

Aviation authorities said it was safe to fly around or under the plume, which poses a significant threat to aircraft, particularly jet engines, but left it to individual airlines to determine what to do.

Qantas acted conservatively, cancelling about 350 flights and disrupting 40,000 passengers, but said the vast majority of the backlog had already been cleared.

In a message to travellers, Qantas chief executive Alan Joyce said the airline did not cancel flights lightly but there was insufficient equipment in Australia to measure volcanic ash density.

"Our policy is not to fly into areas where the concentration of volcanic ash is unknown. Without certainty about the density of the ash, we do not consider it safe to fly," he said.

Volcanic ash which enters an engine can be converted into molten glass as a result of the high temperatures and potentially cause an engine to fail, Joyce added.

"Other risks include windscreens becoming opaque, contamination of cabin air and hydraulic systems and erosion of aircraft parts," he said.