The girlfriend of Sydney siege gunman Man Haron Monis has been found guilty of murdering his ex-wife.

Amirah Droudis stood trial in the New South Wales Supreme Court accused of killing the woman at a unit block in Werrington in April 2013.

The victim, who cannot be identified for legal reasons, was stabbed 18 times before being doused in petrol and set alight.

Monis was charged with being an accessory to murder and was out on bail when he was killed at the end of the Lindt cafe siege in December 2014.

Following a judge-only trial, Justice Peter Johnson said he was satisfied that Monis planned the murder and Droudis carried it out.

Justice Johnson rejected suggestions Monis may have paid someone else to kill his ex-wife, saying he did not have the financial means and that it was a "hot-blooded" murder likely to have been carried out by an amateur killer.

The trial heard Man Haron Monis 'brainwashed' Amirah Droudis into killing his ex-wife. ( AAP: Dan Himbrechts )

"The killing involved a frenzied knife attack with multiple stab wounds being inflicted to the body of the victim, followed by the gratuitous use of fire," Judge Johnson said.

"This crime had the hallmarks of a frenzied attack by an angry amateur killer."

Justice Johnson also said while Monis had previously asked members of the Rebels bikie gang to murder his ex-wife, his requests were not taken seriously and he did not have the money to pay for a professional killing.

Droudis 'enthralled' by Monis, court hears

The court was told Droudis and Monis met after Droudis' mother made an appointment to see Monis as a spiritual healer in 2003.

Droudis later converted to Islam and changed her name from Anastasia to Amirah in July 2008.

She appeared in 11 videos, some of which were posted online, in which she praised Osama bin Laden and expressed happiness about the September 11 terror attacks, the Bali bombings and the Holocaust.

She and Monis were also charged with contacting and causing offence to the families of Australian soldiers killed while serving in Iraq and Afghanistan.

Man Haron Monis was on bail as an accessory for his ex-wife's murder when he carried out the siege. ( AAP Image: Sergio Dionisio )

Justice Johnson said Droudis was "enthralled" by Monis and "adopted uncritically Monis' view of the world with its extreme and perverse features".

"Monis took advantage of [Droudis'] willingness to act at his behest," Justice Johnson said.

A friend of the victim, asked to be known only as Sonia, said she had tears of joy when the guilty verdict was announced and thanked everyone involved.

"The day that she died, everyone's life changed forever," Sonia said.

"I was not expecting something like this to happen.

"I want her to get the death penalty, if we ever had one in this country."

She also said that it was like Monis had "gotten away with murder".