Optus subscribers on a Reddit forum made no effort to hide their annoyance at the hikes, with one user saying his/her plan had been hiked from $30 per month to $49 per month.

"I like many users seem to have gotten the email that the $30/month plan is ending at the start of August, and I'll be shifted over to the $49/month plan. I really can't justify a +60% increase in my monthly phone bill when there is no benefit to me..." this user wrote.

Telstra said in April that it would be raising its prepaid mobile rates in July, with Boost also announcing a rise as it resells Telstra's service.

Contacted for comment on Thursday, an Optus spokesperson said: "As with households and businesses across Australia, Optus is experiencing increases in our costs to deliver services for our customers

"To ensure we can continue deliver great value to our customers, we are streamlining the mobile plans we offer. As a result, we are moving some customers on older plans to more current plans, which may mean they see changes to their monthly bills.

"We’ve communicated these changes to those customers affected, as well as any discounts and data bonuses that will apply to their service.

"Optus continues to invest over $1.5 billion in our network each year to provide our customers with great value plans, super-fast 5G and innovative Living Network features, like Call Translate, Pause and Turbocharge.

"Unlike others in the market, we are not changing the price of the plans currently available."

In May, Telstra announced a rise in post-paid mobile plans as well.

Another Optus user on the same Reddit forum said he/she had contacted the company via online chat and been offered the $30 plan for $34.

"Just go on chat, say you're unhappy about the price increase, ask if there's anything they can offer or you'll be porting your number elsewhere," this user advised.

"They'll transfer you to the Loyalty Team (something like that), who instantly applied an ongoing $15/month credit to my account. Whole thing took about half an hour."

Users of Amaysim are also expected to be hit by similar price increases as this MVNO resells Optus' service. However, Amaysim has not yet written to its users. Update