Strong winds have carried the ash clouds 9 400km across the Pacific to New Zealand since Chile's Puyehue volcano erupted more than a week ago, and they are now moving towards Australia.
Qantas cancelled all flights in and out of the southeastern island of Tasmania and services from Sydney and Melbourne to and from Christchurch, Queenstown and Wellington in neighbouring New Zealand, disrupting about 1 500 passengers.
An earlier flight to Auckland had gone ahead, Qantas said, but services to this city were also at risk of being suspended. The airline was expected to make another announcement later on Sunday.
"We will be providing an update throughout the day to different passengers travelling tomorrow but at this stage it is only flights during today that have been impacted," Qantas spokesperson Olivia Wirth told ABC Radio.
"We believe that it's absolutely the right thing to ground these services. We will put them back up in the air as soon as possible but it's a safety first approach for our customers."
Qantas offshoot Jetstar has also grounded flights due to leave Sunday for Tasmania and New Zealand, as well as flights within New Zealand, but rivals Virgin Australia and Air New Zealand are flying as normal and monitoring developments.
Air New Zealand has said it will 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 on Saturday New Zealand airspace might be affected for at least a week, given that the volcano was still erupting.
Air Services Australia said the cloud could affect air traffic for the next few days, and had already reached airspace in southern Tasmania and the south island of New Zealand.
High ash concentrations
The authority said it was not expected to linger over Australia for more than a day.
"Parts of the ash cloud currently have relatively high ash concentrations clearly visible using satellite techniques," it said in a statement.
Andrew Tupper, head of Australia's Volcanic Ash Advisory Centre, said he expected the ash cloud to move towards Australia's east coast over the next day, although the plumes were breaking apart.
"It's got a very strong satellite signal and it's right up there with the big, big eruption clouds," he said.
"It will keep going. I would suspect it will do a loop of the globe."
Flights to and from Australia are from time to time disrupted by volcanic eruptions in the Pacific, but Tupper said this was the first time in about two decades that an ash cloud was expected to cross the continent.
"It's pretty major," he said.
The volcanic eruptions have already prompted the cancellation of several flights across South America.