At least 16,000 passengers on both sides of the Tasman have now been effected by flight cancellations because of ash from a volcanic eruption in Chile.
The ash is moving over most of New Zealand, Tasmania and southern Victoria and it could cause problems for some days.
The Puyehue Cordon Caulle volcano in Chile has been disrupting travel in Argentina, Uruguay and Brazil, since it began erupting on 4 June.
By midnight on Sunday the ash, at a height of between 20,000 and 35,000 feet, will lie above all of New Zealand bar the tip of the North Island.
The Civil Aviation Authority's Meteorological Manager, Peter Lechner, says it is likely to stick around for most of Monday before drifting back to South America.
He says even if the volcano stopped erupting immediately New Zealand could still be hit by the fallout six days later because the ash can disperse very slowly.
The ash has forced at least four airlines to ground scores of domestic and international flights in Australia and New Zealand.
One of them, Qantas, has cancelled 18 trans-Tasman flights.
A spokesperson, Olivia Wirth, says the situation will be assessed later on Sunday evening.
She says Qantas remains hopeful that the ash cloud will dissipate and that aircraft can fly again, but that will only be when it's safe to do so.
Jetstar has grounded all of its domestic flights, as well as trans-Tasman flights.
It has also grounded flights between Tasmania and Melbourne, Sydney and the Gold Coast.
Jetstar says 6000 passengers in New Zealand have been effected, and 2000 in Australia.
Jetstar says it will decide on Sunday evening whether any flights on Monday need to be cancelled.
Pacific Blue and Emirates have also cancelled domestic and trans-Tasman flights.
Air New Zealand flights still operating
Air New Zealand planes have been flying on Sunday and it says they will continue to do so on Monday, adjusting altitude and routes to avoid the ash.
It says where required, aircraft will fly at a lower altitude of 18,000 feet to remain below the ash or operate on a slightly different course.
The airline says pilots are explaining this to passengers on each flight as needed.