Lufthansa and Air Berlin have questioned the decision to close much of Europe's airspace, saying it is not based on proper testing and their aircraft showed no signs of damage after flying without passengers.

"The decision to close the airspace was made exclusively as a result of data from a computer simulation at the Volcanic Ash Advisory Centre in London," Air Berlin chief executive Joachim Hunold told the Bild am Sonntag newspaper.

"Not one single weather balloon has been sent up to measure how much volcanic ash is in the air."

Lufthansa spokesman Klaus Walter told Bild: "The flight ban, made on the basis just of computer calculations, is resulting in billion-high losses for the economy.

"In future we demand that reliable measurements are presented before a flying ban is imposed."

Lufthansa flew 10 aircraft without passengers from Frankfurt to Munich on Saturday, not as proper test flights but to prepare for air traffic returning to normal, Lufthansa spokesman Wolfgang Weber said.

"The planes were analysed and show no signs of damage, not even the slightest scratch on the cockpit glass, the fuselage or the engines," Mr Wolfgang said.

The aircraft flew at heights of between 3,000 metres and 8,000 metres, "and apparently up until 8,000 metres, there is no volcanic ash," he said.

However earlier Germany's national air safety agency DFS said the country's airspace would remain closed until at least 8:00pm local time on Sunday.

Lufthansa said it was cancelling all its flights until the same time. All the country's 16 airports were paralysed including Frankfurt, one of Europe's busiest hubs.

Qantas calls Australians home

Australian travellers are also likely to remain in travel limbo with Qantas urging its passengers stranded in Asia to return to Australia.

The airline has cancelled its flights between Asia and Europe until at least Tuesday afternoon.

But Qantas spokesman David Epstein says he is not confident that flights to the region will resume on Wednesday.

Singapore Airlines predicts it will be at least another 24 hours before it can resume flying to the region.

Mr Epstein says it is better for passengers to wait in Australia rather than add to the accommodation pressures in Asian stopover destinations.

He says more than 12,500 people have been affected.

"Some of those people have been able to make new arrangements and we're increasingly bringing people back from Asian ports and indeed overnight we've brought back I suppose a further 300 people into Australia," he said.

"And we would encourage more people who are stranded in Asia to consider that option."

- AFP/ABC