Qantas' trans-Tasman flights have been cleared for today.
Its domestic flights from Christchurch are set to resume at 6pm.
Jetstar has resumed all domestic and trans-Tasman flights, while its flights in and out of Christchurch will also restart from 6pm.
Extra flights have been scheduled from some centres to help clear the backlog.
Virgin has cancelled flights scheduled today between Brisbane and Dunedin, Wellington and Brisbane and Christchurch and Sydney.
Ash cloud could remain for months
A director of Sernageomin, Chile's National Service of Geology and Mining, flew over the volcano on Wednesday and told local newspapers Puyehue-Cordon showed similar signs of activity to its 1960 eruption, which lasted for two months.
GNS Science volcanic geologist Graham Leonard, who is analysing the Sernageomin data, said: "There is no guarantee that it will be drawn out. But the overall character of this volcano - its personality - is capable of producing drawn-out eruptions.
"Nobody is forecasting an immediate stop to any activity. Just the fact that it is a large, pyroclastic [explosive] eruption - these things are typically capable of going on for at least weeks, if not months."
About 9pm on Wednesday, an ash plume descended to 3000m above the east coast of the South Island, preventing almost all air travel in the bottom half of the country until yesterday afternoon.
Ben Liley, a National Institute of Water and Atmospheric Research physicist who measured the plume by laser, said the 2000m-thick cloud blocked five-sixths of the light over the South Island.
"It was very optically thick at that time. You can't see anything but the brightest stars when it gets like that."
Last night, the plume had risen to 4000m and had cleared enough for Air New Zealand to resume flights. Qantas, Jetstar, Pacific Blue and Virgin had suspended their domestic and transtasman flights. A higher plume remained at 8200m.
Air Line Pilots' Association technical director Stu Julian said the decision to fly ultimately came down to the pilot in command.
"Passengers should take comfort in the knowledge that in any situation, the pilot will take all factors into consideration and make their assessment based on the latest information."
The Civil Aviation Authority said there had been no reports so far of ash-related damage to aircraft.
If the volcano erupts for months, it does not mean there will be a constant stream of ash over New Zealand. It could alternate between explosive and lava-producing eruptions.
Only the ash ejected into the stratosphere will reach this part of the world.
Sernageomin's most recent report showed the volcano was stabilising and ejecting an ash plume lower (9100m) than its initial burst (15,000m).
