FedEx couriers say they have been locked out after walking off the job amid industrial action expected to affect Christmas deliveries.
The Transport Workers Union says from Monday its members will participate in national rolling four-hour strikes this week.
The parties have been in dispute over pay and conditions for months with a string of strikes this year, as negotiations over a new enterprise agreement stalled again last week.
FedEx has told the union that workers who participate in four-hour stoppages will be locked out for the remainder of the shift and the following full shift, meaning many workers will lose two days' pay per action.
TWU National Secretary Michael Kaine says FedEx's decision provides workers with greater ability to take more disruptive industrial action.
"FedEx is cutting off its nose to spite its face," he said on Monday.
"Management would rather start an all-out brawl with workers at great cost to its business and customers than consider the reasonable solution brought to the table by union members," he said.
While passing drivers honked support for the workers outside the parcel delivery service's Enfield facility on Monday, Australian Council of Trade Unions secretary Sally McManus said they were fighting for things "every Australian deserves" such as job security, superannuation, and wage rises that keep pace with the cost of living.
"The workers who delivered to Australians right throughout the pandemic" are striking because they want "a tiny part" of the profits FedEx generated during the period.
"They're fighting for a basic fair go and that's why we're all standing behind them," Ms McManus said.
Mr Kaine says other parcel delivery companies have come to the table and reached agreements, and FedEx were now "outliers who are refusing to reach a fair deal".
He accused the company's US-based managers of intervening, saying the union had twice come close to reaching a deal before overseas managers "intervened and moved the goalposts".
The TWU says rolling four-hour work stoppages will affect NSW and WA on Monday, be rolled out on Tuesday in Victoria and Tasmania, and continue on Wednesday in Queensland and SA.
The union says further action will be determined by workers who are now able to walk off the job with minimal notice.
In a statement a FedEx spokeswoman said the company was locking out workers "in response to this industrial action to discourage further unnecessary disruptions to our customers and the supply chain, especially as Australia heads into the peak e-commerce holiday period".
FedEx has been contacted for further comment.