Virgin resumed limited flights in and out of Melbourne, Tasmania and New Zealand this morning but Qantas planes remained grounded as a volcanic ash cloud brought chaos to air travel across the region.

Tens of thousands of travellers remain stranded after dozens of flights were cancelled as the ash plume from the Puyehue volcano streamed across the Atlantic and Indian oceans and into Australian and New Zealand airspace.

In developments this morning:

Virgin said its first flight had left Melbourne at about 7:15am (all times AEST).

Virgin said its first flight had left Melbourne at about 7:15am (all times AEST). Some Virgin flights out of Tasmania and New Zealand were due to resume at around the same time.

Some Virgin flights out of Tasmania and New Zealand were due to resume at around the same time. But Qantas said none of its flights would fly in or out of Melbourne until at least 1:00pm.

But Qantas said none of its flights would fly in or out of Melbourne until at least 1:00pm. Qantas cancelled all Tasmanian and New Zealand flights for the rest of the day

Qantas cancelled all Tasmanian and New Zealand flights for the rest of the day Jetstar said flights would remain on hold until midday at the earliest.

Jetstar said flights would remain on hold until midday at the earliest. Tiger Airways said its flights would remain grounded until at least 5:00pm

Airlines are waiting for more information from weather bureaus and the Australian Volcanic Ash Advisory Centre in Darwin to see whether further cancellations are necessary, and are recommending passengers check the status of their flights online before arriving at the airport.

This morning Virgin spokeswoman Melissa Thomson said the airline believed it was safe to resume flights around or under the ash cloud.

"We decided to resume services based on information that we received from the Bureau of Meteorology and also the Volcanic Ash Advisory Centre," she said.

She said other airlines would have to make their own decisions on how and when to restart services.

"I couldn't comment on their processes. We've taken into consideration expert advice, and while safety is always our priority we believe that it is safe to resume services," she said.

"It's something that we will continually monitor."

Qantas spokeswoman Olivia Wirth said around 10,000 Qantas passengers and around 12,000 Jetstar passengers were affected by the shutdown.

"This is an inconvenience, we do recognise that," she told ABC News Breakfast, adding that the situation was "out of our control".

"Our Qantas group policy is that if there is any sign of ash cloud around we won't operate," she told ABC Local Radio.

"The problem at the moment that we have is that we just don't know the density of this ash cloud. There isn't the technology here in Australia to determine that, so we don't know whether it's a thick ash cloud or one that's quite thin.

"So at this stage, if there's any sign of ash we won't be flying."

A spokesman for the Volcanic Ash Advisory Centre in Darwin said any amount of volcanic ash could pose a risk.

"We advise not to fly in the areas that we've warned," he said.

But he said it was up to individual airlines to decide whether or not to fly and said the ash cloud was probably not particularly dense.

"It's been in the air for about a week, so most of the heavy particles have fallen out," he said.

The Australian National University's Professor Richard Arculus has warned disruption to flights could last for days.

"You can see the ash coming," he told AM.

"It has come all the way around the Atlantic and across the Southern Ocean, Indian Ocean and is almost going to do a loop on itself, heading back towards South America.

"So it will take a few days for that to disperse. The question for travellers is what is the density of particles per cubic metre, that's the thing the airlines worry about.

"But when you look at the advisory, you can see this plume coming with swirls on it from our west, from across the Indian Ocean and the eruption is continuing, not as violently as it did when it started on June 4 but it is still continuing to some extent."

Air New Zealand said it would adjust flight routes and altitudes to avoid the plumes, which New Zealand's Civil Aviation Authority has warned will be at 20,000 - 30,000 feet - the cruising altitude for both jet and turboprop aircraft.

The authority said New Zealand airspace may be affected for at least a week, given the volcano is still erupting.