Former Liberal prime minister John Howard has called on the Turnbull Government to provide more detail on religious exemptions from same-sex marriage laws ahead of the country's postal vote.

Key points: No comprehensive legislation protecting religious freedom

No comprehensive legislation protecting religious freedom Attorney-General insists religious groups would be protected

Attorney-General insists religious groups would be protected Howard says "bit rich" of Labor to suggest those against same-sex marriage are bigots

Mr Howard is campaigning for the No vote — to maintain the current definition of marriage between a man and a woman.

Some opponents are worried any change to the Marriage Act could leave religious groups vulnerable to action under Australia's discrimination laws.

There is no comprehensive commonwealth legislation protecting religious freedom or prohibiting discrimination on the basis of religion, however some states have introduced their own legislation.

Attorney-General George Brandis insisted they would be protected, but Mr Howard told AM the Federal Government needed to provide more detail.

"I think the Government does need to define what steps it will take to ensure that state governments don't withdraw exemptions in discrimination acts … I think that's very important," he said.

"I don't think it's good enough, as some have said, that, 'Well, we'll deal with that issue after the vote'.

"They've got to deal with that issue before the vote. Saying, 'we'll deal with it afterwards' is the equivalent of saying in an election campaign: 'I'll spend $50 billion on roads but I'll tell you after the election how I'm going to pay for it.'

"If you tried to do that in a modern election campaign you'd be laughed out of court."

The private member's bill proposed by Liberal MP Dean Smith earlier this year extended religious freedoms to all ministers, while service providers — such as bakers and florists — needed to prove a link to a religious body to object.

However Mr Howard said the Government needed to consider protections outside of the ceremony.

"I think we need more detail, more assurances, more specificity, and the detail in Dean Smith's bill only related to the putative marriage ceremony, and there's a lot more to it than that," he said.

"They are areas the Government does need to address before the vote takes place."

'Bit rich' of Labor to use bigot label

In 2004, the Howard government changed the Marriage Act to include a definition of marriage as the union of a man and a woman.

The Coalition pushed the legislation through Parliament without a postal ballot, however Mr Howard said it had the support of the Labor Party.

"The reason my government put the definition in the act back in 2004 — with the support of the Labor Party, let me stress — was that we didn't want the courts deciding it," he said.

"We took the view if this issue ever be other than what everybody understood it to be then it should be determined by the people through the parliament. We put up an amendment to define it and that was supported by the Labor Party.

"That's only 12 or 13 years ago, so it's a bit rich for the Labor Party now to say anyone against same-sex marriage is a bigot."

Nobody to blame on citizenship

The former prime minister cautioned the public to react "calmly" to the citizenship saga engulfing Parliament.

"There's nothing you can do about it, it's the law working its way through," he said.

"My view about this is nobody is to blame. I think trying to apportion blame and people running around saying, 'Oh you knew about this, you should have known', I think that's silly."

He was asked whether the constitution should be changed.

"There are more important areas where the constitution should be changed but I also know it's hard to change, and in most cases that's not a bad thing," he said.

"I don't think it's a crisis for democracy and I'm frankly surprised people are running around trying to apportion blame."

Editor's note: An earlier version of this story stated that Mr Howard said the Government needed to consider protections outside of the ceremony – such as whether religious schools would be allowed to ban enrolments of children of married same-sex couples. This was incorrect and has been removed from the story.