Former Australian Prime Minister Tony Abbott has told an anti-abortion rally that a proposed bill seeking to decriminalise abortion in NSW enables “infanticide on demand”, while Nationals MP Barnaby Joyce labelled it "animalistic", days before the proposed legislation returns to the state’s parliament.

The Sunday afternoon rally, which took place at Sydney’s Hyde Park, came a day after abortion rights supporters gathered in the same park to put forward their support for the historic legislation, which if passed, will remove abortion from NSW’s 119-year-old Crimes Act.

“We elect sensible governments of the centre-right to serve us, in practical ways, not to engage in social engineering,” Mr Abbott said.

An anti-abortion crowd has gathered in Hyde Park and is chanting “ditch the bill” and “stand for life” ahead of a debate in NSW parliament this week @SBSNews pic.twitter.com/xFJvCMwJzN — Hannah Sinclair (@HanSinclair) September 15, 2019

“It [the bill] is based on a lie. It is not about decriminalising abortion. Whether we like it or not abortion has been decriminalised in this state for more than 40 years.

“It is a licence for sex-selection abortions, it is a licence for late-term abortions, it is effectively infanticide on demand … You don't have to believe in anything other than the basic decency of every human being to think that we should never have infanticide on demand.”

READ MORE The Australian women fighting to change a 119-year-old law that makes abortion illegal in NSW

As Mr Abbott left the stage, the crowd chanted “bring him back”.

The former prime minister appeared alongside Nationals MP Barnaby Joyce and senior religious figures, including Catholic Archbishop of Sydney Anthony Fisher and Anglican Archbishop Reverend Glenn Davies.

Mr Joyce, who is the federal Member for New England, described the debate over the bill as "the slavery debate of our time".

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"There is not one person here who was not born ... there comes a time when there are two people, and two people have rights, and those rights must be respected otherwise society is destroyed," he said.

"If you want to kill someone, how do you do it? You dehumanise them, you give them another word, you don't call them a person ... well, I'll call them a fetus."

Archbishop Fisher kicked off the event with a prayer, before delivering a statement describing the proposed legislation as the “kill bill”.

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“They call themselves pro-choice, but what choice do the promoters of the abortion bill offer the unborn? Death, no reason given, up until 22 weeks,” he told the crowd to boos.

“To women, the kill bill says you have no choice but to abort … the ‘kill billies’ are bullies who want you to have no choice.”

If successful, the new bill would allow for abortions up to 22 weeks to be performed by a registered doctor. For terminations after that point, the consent of two doctors would be needed.

Currently, for someone seeking an abortion after the 20-week point of pregnancy, there are no options in NSW.

Former Prime Minister @HonTonyAbbott gets a huge round of applause ahead of his address. He says the NSW Liberal government is supporting “the most radical abortion laws in this country” and that the bill will lead to “infanticide on demand” pic.twitter.com/QST9EgsBex — Hannah Sinclair (@HanSinclair) September 15, 2019

Attendees at Sunday's rally carried signs with slogans such as “everybody deserves a birthday” and pictures of babies alongside the phrase “let me live”.

“I don’t think this new law that’s been proposed is the best thing for NSW, we’ve waited a long time for this situation to be addressed and it doesn’t protect women,” Fiona Carolan, 45, told SBS News from the rally, where she had brought her three kids.

“We’re not doing enough to support women who are facing an unplanned pregnancy. Women need to be given more options, rather than fewer options.”

Next week, the Reproductive Health Care Reform Bill 2019 will return to the upper house, where MPs will consider a number of proposed amendments before voting on the bill, weeks after it was passed in the lower house.

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At the rally on Saturday, abortion rights supporters urged the government to dump the proposed amendments, arguing that many would actually make the situation for women worse than it already is.

The activists chanted "no amendments" and "trust women support the bill” before police intervened when a small group of anti-abortion protesters came face-to-face with abortion rights supporters.

Mr Joyce, who is a federal MP, has previously come under fire for his involvement in the NSW abortion debate after he sent out pre-recorded robocalls urging residents to oppose the bill.

In the messages, he said the bill would allow legalised abortion, for any reason, "right up to the day of birth".