Sydney siege gunman Man Haron Monis was released from custody on bail six days after controversial changes to the state's bail laws came into effect in May.

The NSW government has called for an investigation into why Monis "slipped through the cracks" of the judicial system and was released from custody.

It follows calls for the government to fast-track stricter bail laws in light of the tragedy.

Monis, who was shot dead by police in the early hours of Tuesday morning, was before the courts on two separate and serious matters: more than 40 sexual assault charges involving seven alleged victims and as an accessory to the murder of his former wife.

NSW Attorney-General Brad Hazzard said on Tuesday that stricter bail laws would take effect at the end of January, and it was "very unlikely" that Monis would have been released on bail had those rules been in place.

"This government changed the Bail Act to ensure greater safety for our community. It was changed to ensure that offenders involved in serious crime will not get bail," he said.

The Herald can reveal that Monis was initially refused bail on April 14 after being charged with three sexual assault offences allegedly committed against a woman in 2002.

He appeared in Kogarah Local Court under the name Mohammad Hassan Manteghi where magistrate Christine Haskett refused bail.

At the time, Monis was already on conditional bail granted on December 12, 2013. He had been charged with being an accessory before and after the fact to the murder of his former wife Noleen Hayson Pal in April 2013.

Monis did not apply for bail when he appeared in Parramatta Local Court on April 16 and continued to be remanded in custody.

On May 20, the state government introduced sweeping changes to bail laws, removing the decades old presumption against bail for a suite of serious offences.

Magistrates and judges were now to use a two-step test as to whether the accused posed an "unacceptable risk" of reoffending and whether the risk could be mitigated by bail conditions.

On May 26, Monis was granted bail at Parramatta Local Court by magistrate Joan Baptie.

Ms Baptie said she had identified two "unacceptable risks" – that Monis may endanger the safety of victims, individuals or the community, and that he may interfere with witnesses or evidence.

However, the magistrate granted him conditional bail, and reinstated the conditions placed on him in December 2013, including daily reporting to Campsie police station, surrendering his passport, not going within 500 metres of a point of overseas departure and not contacting "the prosecution witness".

Monis had to find a person to deposit $10,000 cash and was ordered to reside at an address in Wiley Park.

His partner, Amirah Droudis, was charged with Ms Pal's murder and was granted bail on the same conditions, although with a $100,000 surety.

Following the grant of bail in a series of high-profile cases including those of bikies Mahmoud "Mick" Hawi, Hassan "Sam" Ibrahim and accused wife-killer Steven Fesus, the Baird government announced in August that the laws would be toughened.

The changes come into effect on January 28, 2015; they include a revised test under which an accused person who is assessed as an "unacceptable risk" will be refused bail.

For serious offences, the onus will be on the accused to "show cause" why their detention in custody is not justified.

On October 10, Monis was charged with a further 40 sexual assault charges against six women. They included 22 counts of aggravated sexual assault, 14 counts of aggravated indecent assault, one count of aggravated act of indecency, one count of sexual assault and two counts of assault with act of indecency. The women were visiting him at Wentworthville where he was offering services as a "spiritual healer". His bail was continued by magistrate Dorelle Pinch.

He was due to appear in Penrith Local Court on Friday, December 12, but this date was vacated and changed to February 27, 2015.

On Tuesday, Mr Hazzard said the government would ask state and federal agencies to examine "how this offender slipped through the cracks".

"How did this offender not come to the attention of state and federal agencies ... what exactly did they miss?" he said.

NSW Premier Mike Baird later said bail laws may be tightened further following the investigation, adding the government would "do anything we possibly can" to ensure public safety.

Julia Quilter, senior lecturer at the University of Wollongong's School of Law said, under the "unacceptable risk" model, the magistrate hearing Monis' bail application could have refused bail, but chose to grant it.

"In retrospect we should have had alarm bells ringing about his unacceptable risks," she said.

"But the magistrate can't gaze into a crystal ball and know what's going to happen in the future."

Former NSW DPP Nicholas Cowdery said the new amendments would not have made any difference in Monis' case because the magistrate and the police were not aware of the "dark and evil" thoughts he must have been secretly harbouring.

"You can't legislate to deal with that. It doesn't matter how much you muck around with the laws, there are still going to be occasions - hopefully rare - where the justice system cannot see into the deep psyche of such a person."

Bail laws weakened: Hadley 

On Tuesday, 2GB presenter Ray Hadley criticised former attorney-general Greg Smith for the Bail Act changes.

Addressing listeners, Mr Hadley said: "What we needed to do with the old bail laws was toughen them up. But what did we do? We weakened the bail laws."

Comment has been sought from Mr Smith.

Hadley said on radio that Mr Smith had threatened to sue him for defamation over his comments.

There were calls on Tuesday for the government to bring forward the changes in the law following the Martin Place incident.

The government has said that police and the judiciary were being trained in the new system.

Mr Hazzard said the government had received expert advice that rushing the changes would "be creating more problems".

"It is frustrating … but we must accept the advice from our professionals," he said.

On the Today show, Hadley said Mr Baird had told him the delay in implementing the laws was due to "administrative bungling and problems".

At a press conference on Tuesday morning, Mr Baird was asked whether he was concerned Monis was out on bail.

"I'm concerned that there was a vicious, horrendous attack that has taken place in the heart of our city, and there are many questions that will come in the coming hours, days and weeks," Mr Baird said.

"What I can assure you is I will answer every single one of them.

"We will get to the bottom of events and we will do everything possible to ensure we do not see happen again."

More than 19,700 people have signed a petition on change.org calling for "stronger bail laws that would have kept this dangerous man behind bars right now".

"The public deserves to be protected from violent criminals" the petition, started by Sarah Langston, says.