Qantas has been accused of locking out ground staff preparing to take part in a strike which will affect more than 6,000 domestic air passengers today.

Baggage handlers, caterers and freight staff are stopping work for four hours from 7:00am AEST as part of an ongoing row over pay and conditions.

Qantas says the strike will affect more than 6,000 passengers at capital city airports across the country.

It has cancelled or delayed 55 flights and warns there could be more problems over the next 48 hours due to administration bans.

This morning the Transport Workers Union accused Qantas of locking out baggage handlers preparing to take part in the strike in Melbourne, Brisbane and Sydney.

The TWU says its members want a 15 per cent pay rise over the next three years, job security, more superannuation, and the same rates for contract workers.

The union says this morning's lock-outs will only make the situation for passengers worse.

"Our concerns about minimising disruption for the travelling public have been thrown out the window by Qantas," the union's Victorian state secretary Wayne Mader said.

"They don't care, this is a full-on attack against their own workers."

But Qantas has denied locking workers out, saying it had asked staff not to turn up for work if they were planning to take part in the industrial action.

Sorry, this audio has expired Day of delays for Qantas passengers

"This was not a lockout," Qantas spokeswoman Olivia Wirth said.

"What we have done is we have got a contingency workforce in place around Australia. We needed to do that in order to keep the operation running.

"There are some workers that have turned up before the strike action has taken place but you simply can't turn an airline on and off, it doesn't work like that."

There will be more delays to flights this afternoon when engineers in Sydney walk off the job for an hour.

The strike comes after last-minute talks between the union and Qantas at Fair Work Australia failed to reach a resolution on Monday afternoon.

"We're expecting that members throughout the country will be taking industrial action tomorrow, (for) up to four hours," TWU national secretary Tony Sheldon said.

"This is a position that they've been forced into by Qantas's lack of good faith and negotiations."

The West Australian branch of the Transport Union says it understands passengers' frustration.

"But they've also got to understand the frustration of the Qantas employees who for the last 6 months have been trying to reach an agreement," organiser Rick Burton said.

"Meanwhile the CEO of Qantas has got a $2 million pay rise, taking him up to $5 million, and we haven't been offered a cent."

The bulk of the affected flights are morning services from Melbourne to Sydney. International flights will not be affected.

A Qantas spokesman said the airline was putting on additional aircraft and bigger planes. All passengers affected by the action would be informed of alternative arrangements by text message, he said.

"All passengers who have had flights cancelled, they will all get on an aircraft," he said.

"We're asking passengers to be patient where they can.

"We ask them to turn up to the airport as planned but to obviously understand that it's not going to be a normal day of travel tomorrow."

The TWU says Qantas workers who will replace their colleagues during the strike have the right to say no.

Mr Sheldon says there are grounds for strike breakers to refuse to fill in for shifts.

"If you're not properly trained, if you're not properly skilled, you shouldn't just think it's a day to follow the employer's behest," he said.

"You should say to them, if you haven't got the occupational safety training, you haven't got the right skills, then you have every right to turn around [and] refuse those jobs."