Porter has offered to give evidence under oath and challenged the public broadcaster to run a truth defence, which would transform the legal action into an independent inquiry.
The legal action is also aimed at a report aired in November that alleged he was a misogynist and sexist.
His lawyers have accused the ABC of running a trial by media and Milligan of acting with malice, damaging Porter’s reputation.
“If the ABC and Ms Milligan wish to argue the truth of the allegations, they can do so in these proceedings,” lawyer Rebekah Giles said in a statement on Monday.
“Under the Defamation Act it is open for the ABC and Ms Milligan to plead truth in their defence to this action and prove the allegations to the lower civil standard.”
Giles and colleague Sue Chrysanthou together represented Brittany Higgins in her settlement against Defence Minister Linda Reynolds.
They argue the attorney-general has been subjected to several weeks of trial by media without regard to the presumption of innocence, rules of evidence or proper disclosure of the material said to support to allegations.
“The trial by media should now end with the commencement of these proceedings,” Giles said.
“The claims made by the ABC and Ms Milligan will be determined in a court in a procedurally fair process. Mr Porter will have and will exercise the opportunity to give evidence denying these false allegations on oath.”
The defamation action comes days after a former boyfriend of the woman who accused Porter of rape offered to give evidence about his conversations with both parties about the allegations.
More than 80,000 people have signed a petition calling for Porter to be suspended as attorney-general over the rape allegations he has denied.
The prime minister remains under sustained pressure as he continues to insist police have dealt with the historical allegations and no further action is required.
Porter has taken indefinite mental health leave but Scott Morrison is adamant he will be welcomed back into cabinet.
Finance Minister Simon Birmingham rejected suggestions the government was out of touch with community expectations.
“I think Australians do expect that proper legal processes should be followed in relation to addressing such allegations,” he told ABC radio.
“Those legal processes have been deployed by the NSW Police as they’ve outlined. Of course, there are also legal processes in relation to the tragic death (of the woman involved) and those processes remain under way in South Australia, and we should let them run their course.”
Porter’s former wife was among thousands of West Australians to march across Perth on Sunday in protest against gender discrimination and violence.
Thousands more are marching on Parliament House in Canberra on Monday, with rallies also taking place in Adelaide, Melbourne, Sydney and Brisbane.
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