Australia Post's new regime has started, with the standard letter stamp costing $1 and longer mail delivery services.
Sending a standard letter in Australia will now cost $1 and take two days longer to deliver.
The stamp price hike from 70 cents came into effect on Monday after the Australian Competition and Consumer Commission approved the increase in November.
The concession price for 5.7 million Australians will stay at 60 cents and greeting cards at 65 cents.
Customers took to Australia Post's Facebook post to complain about the changes.
"I don't understand why, if letters are able to be delivered within 4 days, why the regular ones will take longer? Why not just send them all the same way? Where's the logic? That's a strange system," Sonya Baker posted.
Other customers said the price increase would make the company less competitive.
"$1 to send a letter. What a joke! 40% increase on passage stamps. Goodbye Australia Post! Way to compete, NOT!," Karen Rumble said in her message.
"A 80 cent price hike for the same service. What a disgrace. Auspost, please let smarter company run our postal service. You're un-Australian," one Facebook user Michael Buns said.
Customers will be able to speed up their letters by affixing a 50 cent priority label next to the standard $1 stamp.
Australia Post has said the measures are essential after it lost $222 million in 2014/15 due to tumbling demand for its mail business, with Australians increasingly switching to digital options such as paperless billing and email.
The two-speed letter service, with regular post two days slower than a more costly one-to-four business day priority service, was approved by the federal government in March.
The changes bring the cost of posting a letter closer to that in countries including the much smaller UK, where three-day delivery of a standard letter costs 54 pence ($1.12).
