One in four jobs at Australia Post face the axe under its new delivery model, the postal worker union says.
But the postal agency has described the union's claim as speculation.
The Communications, Electrical and Plumbing Union wants parliament to block the changes when it meets this fortnight.
Australia Post was forced to extend delivery times and redeploy workers to other departments in April.
The changes were made to cope with unprecedented demand during the coronavirus pandemic.
Workers also face a pay cut of up to 30 per cent while Australians would have to wait longer for their mail, the union said on Tuesday.
Australia Post says the concerns are speculative.
"No postie that is directly impacted by the implementation of the (alternating delivery model) will be forced to accept a redundancy," it said in a statement on Monday.
"Australia Post also has no plans to cut posties' take-home pay."
It said any changes to employee working arrangements or enterprise agreements would be made in consultation with workers.
Under the new model announced last month, 2000 motorbike posties would trade bikes for vans or move into warehouses to cope with the increased load.
Priority letter services were suspended, letter deliveries were moved to every second day and five days were allowed for intrastate posting.
The changes were meant to be in place until June 30 next year and lifted after a review.
"These changes have been requested to enable Australia Post to continue to offer important community services and remain sustainable for the future," it said.
Australian Associated Press