Australia’s former top statistician says the Bureau of Statistics should not carry out the same-sex marriage postal vote, labelling it a “bad look”.

Bill McLennan, Australian Statistician and head of the Australian Bureau of Statistics between 1995 and 2000, said he doubted the ABS had the authority to conduct the vote.

The Turnbull government confirmed on Tuesday that it would ask the ABS to conduct a voluntary postal plebiscite between September 12 and November 15 provided the Senate rejected its preferred option – a compulsory attendance plebiscite.

The process would cost up to $122 million and ask a single question: “Do you support a change in the law to allow same-sex couples to marry?”

Mr McLennan warned the process could further damage the ABS, which came under fire last year for its handling of the national census and said he had heard “alarm bells” upon learning of the government’s plans.

“I’d be very wary if I was the Australian statistician. It’s not his job,” Mr McLennan told The New Daily.

“If the ABS ran this and it was a disaster, with all the political brouhaha going, you’d have to expect there’s a significant chance it would [break the ABS]. I don’t think anyone would dispute that.”

Mr McLennan, also a former head of the UK’s stats agency, said that he believed if the ABS-led poll was “challenged in a court, it would go down”.

“I don’t think the ABS has the authority to run the plebiscite to be truthful,” he said. “That’s my gut feeling.”

The Turnbull government has announced a postal vote on gay marriage. Photo: AAP

The government was confident on Tuesday it could win a High Court challenge, arguing that the Finance Minister had the power “to make appropriations under certain circumstances of up to $295 million”. That would easily cover the $122 million cost of the postal vote, but the government would have to prove an “urgent” need for the expenditure.

It has declined to release legal advice on the issue, claiming this is common practice for all governments.

Mr McLennan’s doubts centred on the role of the ABS itself, saying the “Minister can direct the ABS to collect statistics”, but that a “plebiscite is not statistics”.

“The ABS Act is quite specific … it says there’s got to be statistics for publication. There’s no way in the world you can say the plebiscite is statistics. It’s just getting a single number. It’s not statistics about who said yes, and who said no, and whether they were men and women or above 35. It’s not a statistical issue. It’s just a vote.”

Noting its difficulties with last year, Mr McLennan called into question the ability of the ABS to conduct a plebiscite, which would be its second-biggest project behind the census.

“Running a plebiscite off an electoral commission list is a different job to [running the census]. You can’t draw a line from one to the other but you have to start thinking if their managerial capacity is up to it.”

His comments come as same-sex marriage advocates seek new legal advice on the constitutionality of the ABS conducting the postal ballot.

“It’s too early to have a confirmed view on exactly whether this carries the same legal risk as the AEC, obviously it’s a different agency,” Equality Campaign co-chair Anna Brown said on Tuesday.

Finance Minister Mathias Cormann argued that the Whitlam government’s decision to ask the ABS to survey 60,000 Australians by telephone poll to ask their opinion on the national anthem served as a precedent.

But Ms Brown said it was unclear how the ABS Act would cover such a “extraordinary proposal”.

“This is not a survey of a handful of households, or even 60,000 households as was the example pointed to this morning by the government.”