With all states on school holidays from tomorrow, people travelling to see relatives or escaping to warmer climes can expect to see delays at airports.

Today is expected to be the busiest day at some airports since the start of the COVID-19 pandemic, with school holidays starting in South Australia and Tasmania — and ending in Queensland and Victoria.

NSW is halfway through its school holidays.

At Adelaide Airport, 30,000 people are expected to pass through the terminal today, which is busier than before the pandemic and higher than the previous peak over Easter.

The airport's managing director, Brenton Cox, said the coming Mondays and Thursdays were also expected to be busy, topped by July 22 — the last Friday of the South Australian school holidays.

"There's seven days over the next three weeks where we are going to be close to 30,000 people a day running through, which is busier than before the pandemic and busier than Easter, where our busiest day was 26,000," Mr Cox said.

The airport recommends travellers arrive two hours before domestic flights and three hours before international flights.

Dozens of flights cancelled this week

Thirteen flights in and out of Adelaide Airport were cancelled on Wednesday and then nine yesterday.

More than a dozen flights were also cancelled at Melbourne Airport on Thursday, including because of weather disruptions for passengers heading to flood-hit NSW.

Flights delays and cancellations were reported in Brisbane, Sydney and Perth.

Adelaide travel agent Phil Hoffmann said his company was advising customers to "be patient" and expect changes to flight times.

He said today was "going to be the test".

Most airlines would try to put passengers on the next available flight rather than offer refunds, he said.

"It's happening — we don't have a lot of control over it," Mr Hoffmann said.

"We can only sort of say to people, 'Be patient about it.' Hopefully they can get on to the next flight."

However, he said some people were being put on flights up to six hours later than their original one, and some flights were only announced as cancelled 10 minutes before their scheduled departure.

"Things can change, cancellations are happening, which I often find annoying because they're happening so close to the departure," he said.

"Surely they know well beforehand."

Airlines say demand higher than ever

Virgin Australia said the number of travellers flying this school holidays was 15 per cent more than 2019 levels and was "significantly higher" than during the Easter break.

"Airports and airlines globally are experiencing huge demand as travellers return to the sky as pandemic restrictions ease," a spokeswoman for the company said.

Virgin Australia customers can get a refund or travel credit if a suitable flight is not available to replace their booked one.

Passengers whose flights are delayed overnight can receive $220 for hotel accommodation, $50 for meals and get refunded for the cost of airport transfers and "reasonable personal items".   

Staff numbers also an issue

Transport Workers' Union South SA/NT secretary Ian Smith blamed the flight cancellations on staff shortages that stemmed from airlines sacking thousands of workers at the start of the pandemic.

He said staff numbers were still way below what they were at the start of 2020 and many workers had become contractors not directly employed by airlines.

"They're the security screeners, the cleaners of the planes — all those sort of things — and these people have an impact on whether planes get off the ground," he said.

"Pilots themselves and cabin crews have hours limits that they're allowed to work, and if a plane is late leaving [or] late arriving, it doesn't get off the ground because the baggage isn't sorted out.

"And people don't want to work in airlines — we've even got pilots we believe who have left the industry completely."

He said it was "obvious" the problem would come up once travel returned to previous levels.

"Qantas itself could fix the baggage handling by re-employing the 1,600 or 2,000 workers they sacked illegally," he said.

"There's a lot of them that would love to come back and work for Qantas."

Mr Hoffmann said the travel industry in general was finding it hard to get workers because of job insecurity.

"We've noticed in our industry as well I could put on 15 to 20 new senior consultants back on tomorrow, but we can't get them," he said.

"They don't want to come back to an industry that has been shaken so bad by COVID."