Opposition Leader Peter Dutton says voters should be able to validly use a tick or cross on their ballot paper for the Indigenous voice referendum.
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Prime Minister Anthony Albanese will announce the date of the referendum in Adelaide next week.
The voting paper will require the word 'yes' or 'no' to be written in a box. This will be made clear in instructions issued by the Australian Electoral Commission.
But Electoral Commissioner Tom Rogers unintentionally triggered a debate after telling Sky News "a tick will be accepted as a formal vote of 'yes', but a cross will not be accepted as a formal vote".
Mr Dutton, who backs a 'no' vote, said he would write to Mr Rogers seeking clarification.
"If a tick counts for 'yes', then a cross should count for 'no' - it's as clear as that," he told radio station 2GB on Thursday.
"Otherwise, it gives a very, very strong advantage to the 'yes' case."
Mr Dutton said he was open to supporting government legislation to clarify the issue ahead of the referendum, which is expected on October 14.
A government source said it wouldn't "respond to fantasy and fiction" and pointed to Mr Dutton and the Liberals having worked with Labor to pass the referendum machinery bill through parliament to modernise the process.
"The process of casting a ballot for a referendum will be the same as a general federal election," they said.
"As with all general elections, our system focuses on making sure votes are counted wherever possible."
The Australian Electoral Commission said the formal voting instructions for the referendum are to clearly write 'yes' or 'no' in full and in English.
The commission expects the instructions to be followed, as they were in 1999 when the informal vote rate was 0.86 per cent.
The AEC has long-standing legal advice about ticks and crosses.
As well, "savings provisions" give the commission the ability to count a vote where the instructions have not been followed but the voter's intention is clear.
A number of government and business forms allow a cross or "check mark" to be used to indicate a positive decision, leaving it open to interpretation or challenge by a polling booth scrutineer.
A tick may also be open to interpretation and may not count, depending on just how clear that mark is on the ballot paper, as would a 'Y' or an 'N' written in the box.
Scrutineers will be present through the count on referendum day to observe the process.
Australian Associated Press
