A key Liberal party figure has agreed polling day signs in two federal Melbourne seats were meant to give the impression they came from the independent electoral commission.

The Chinese-language corflutes were posted at polling places including in Treasurer Josh Frydenberg's seat of Kooyong and fellow Liberal MP Gladys Liu's seat of Chisholm on 18 May, saying the "correct" way to vote was to put the Liberals first.

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Failed Kooyong candidate Oliver Yates is challenging Mr Frydenberg's re-election, and climate campaigner Vanessa Garbett the election of Ms Liu, on the grounds the purple and white corflutes, in colours long-used by the Australian Electoral Commission, were illegal.

"You intended to convey the impression that this was an AEC corflute, didn't you?" Lisa De Ferrari SC, acting for the challengers, asked former Victorian Liberal Party director Simon Frost in the Federal Court in Melbourne on

Wednesday.

Mr Frost, now an advisor to Mr Frydenberg, took a long pause before replying: "It was similar to the AEC colours, yes."

"So the answer to my question is 'yes'?" Ms De Ferrari pressed.

"Yes," Mr Frost replied.

It came after he said the messages on the signs, displayed at seven electorates including Kooyong and Chisholm - in both simplified and traditional Chinese script - were different to what had been approved.

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"The translation was not as I had given ... as it was re-translated back through the media," Mr Frost told the court.

Ms De Ferrari said both Mr Frydenberg and Ms Liu knew about the corflutes and allowed them to be displayed.

Mr Yates and Ms Garbett are seeking to void the MPs' elections, which their lawyer concedes is a "drastic measure".

"The principle, we say, is also important and too important to have those considerations of practical inconvenience take over," she said.

The AEC previously denied the posters were likely to mislead or deceive voters into thinking they were official instructions from the commission.

It urged the court to throw out the cases.