Mining billionaire Clive Palmer is backing a last-minute Federal Court bid to determine how ticks and crosses are recognised in this weekend's Voice referendum.
It comes after the court dismissed an earlier effort by United Australia Party senator Ralph Babet and Palmer, who is chairman of the party, to have "X" votes counted as a No.
An appeal over the decision is being heard by three Federal Court judges in Sydney on Monday.
Barrister Luke Livingston SC argued ticks should not be counted as a formal response to the ballot but if they were, a cross should be similarly recognised.
"If the court concludes that the intention to vote Yes is the only plausible explanation for the use of a tick, we say it would follow that an intention to vote No is the only plausible explanation for the use of a cross," he said.
The Australian Electoral Commission has stated a tick in the box will count as Yes, backed by decades of legal precedent across six referendums.
"The legal advice provides that for a single referendum question, a clear 'tick' should be counted as formal and a 'cross' should not," the AEC said in August.
Livingston said there was a plausible argument a tick should constitute a "non-response" to the ballot question.
"This is a short ballot paper, it has very few words, and they're very clearly expressed," he said.
"For somebody to put a tick on such a ballot paper that so plainly asks for Yes or No, is we say plausibly explained by the voter not reading and not understanding the ballot paper."
Livingston also argued in a system of compulsory voting, a tick may be taken as someone literally treating it as a "tick-the-box exercise" and should in that case also be taken as a non-response.
"No one can say with sufficient certainty that a tick… conveyed with unmistakable clarity the voter's intention to vote Yes," he said.
How Australia voted in all 44 of the nation's referendums
The October 14 referendum ballot will present a proposed law to alter the constitution "to recognise the first peoples of Australia by establishing an Aboriginal and Torres Strait Islander voice".
Voters will be asked: "Do you approve of this proposed alteration?"
To pass, the referendum needs more than half of all voters and four of the six states to vote in favour of the proposal.
The appeal hearing is continuing on Monday.
