Australia's two major airports are facing up to 48 hours of disruption as the ash cloud from a Chilean volcano drifts across the south of the country.
Qantas and Virgin have cancelled all flights into and out of Sydney and Melbourne. Adelaide airport has been shut and Canberra flights also hit.
Last week, tens of thousands of people were stranded as airlines grounded flights, and now the ash has returned.
The plume is said to be too low this time for airlines to fly under it.
The ash cloud from Chile's Puyehue-Cordon Caulle volcano is circling the Earth for a second time.
Last week, some airlines, such as Virgin Australia, flew below and around the ash cloud, but this time they have been advised against doing so.
The ash cloud is reported to be hovering at between 20,000 and 40,000ft (6-13km).
Qantas said its policy was not to fly below the ash cloud, because it brought risks that it was simply not prepared to take.
"We estimate that we will be cancelling in excess of 200 flights on Wednesday," Qantas spokeswoman Olivia Wirth told reporters.
"The experts say we simply won't be able to operate in this situation."