All medications on the Pharmaceutical Benefits Scheme (PBS) would be $10 cheaper under a re-elected Coalition government, the Prime Minister will announce on Saturday.

The election promise would cost the federal government $150 million a year, and see the maximum price of a PBS medicine reduced from $42.50 to $32.50.

It is the second time in a little over a month that the government has made the pledge, having first done so accidentally last month.

Prime Minister Scott Morrison said the measure, which would begin at the start of next year, would benefit up to 19 million Australians annually. But it is now going to begin later than previously flagged.

"This is the single most significant change to the cost of and access to medications since the PBS was introduced more than 70 years ago," Mr Morrison said in a statement on Friday.

"Millions of Australians will soon save $10 per script for common medications, which means those taking one medication a month could save $120 a year, or those taking two medications a month could save $240 a year.

"Because of our strong economic management, the Coalition is winding the clock back on the cost of medications, reducing the cost per script to 2008 prices."

The measure was previously accidentally announced by two frontbenchers as part of the budget.

It appeared to be a late omission from the budget, but Assistant Treasurer Michael Sukkar and Financial Services Minister Jane Hume both referenced it in parliament.

"The amendment will reduce the pharmaceutical benefits scheme general patient category by $10 from the current amount of $42.50 to the new amount of $32.50 commencing on the first of May 22," Mr Sukkar said in parliament.

It was then revealed the Hansard had been changed to remove the error.

"Both Mr Sukkar and Senator Hume told the parliament that the general PBS co-payment was being reduced," senator Katy Gallagher said during an estimates hearing.

"That's not now what's recorded in Hansard. They were told general patients would pay no more than $32.50 in out-of-pocket costs. That's now not what Hansard records."

"Senators correct the Hansard all the time," president of the Senate Slade Brockman said later in the hearing.

On the ABC's Insiders that week, Treasurer Josh Frydenberg would not answer questions about the mistake. 

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"What Michael was talking about actually was the 2.4 million people that will benefit with [a] reduction in getting multiple scripts," Mr Frydenberg said.

"Was this $10 price cut taken out of the budget? Was it?" Insiders host David Speers asked.

"No, what I'm talking about is there was 2.4 million people benefiting from reducing … the safety net," Mr Frydenberg said.

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