By Paul Gregoire and Ugur Nedim
Lucas Delaney and his sister Gabrielle lived together in a detached flat on a property in the Sydney western suburb of Cambridge Park in June 2020, while their brother, Damien, and his wife, Ms Tomkins, lived in the main house on the property, and a cousin of the siblings, Jayleb, lived in another unit on the same grounds.
Jayleb had visited Lucas and Gabrielle in their flat after returning home from work on 3 June 2020, and everything appeared normal. However, sometime after Jayleb left, Lucas repeatedly struck Gabrielle in the head with a hammer, with the extensive fractures to her skull and face proving fatal.
Lucas then proceeded to roll Gabrielle’s body in a blanket on his bed and took an armchair and placed it upside down on top of the body to conceal it. He then went and told Jayleb he was going out for a while, and when his cousin said he might visit Gabrielle, Lucas told him that his sister had gone out to visit a friend, and he told his cousin there was no need for him to attend to their dogs.
After the killing, Lucas went to a friend’s house to smoke methamphetamine, and on the following day, he checked into a hotel. He then transferred $5,300 from his sister’s bank account into his own and withdrew $4,000.
Jayleb and Tomkins were concerned as to Gabrielle’s whereabouts on that same day, with the latter checking her flat to find the dogs were locked inside, but she left without discovering the body.
The offender continued checking in and out of hotels and taking drugs, until he admitted himself to Cumberland Hospital due to his drug use on 7 June. His relatives entered the flat several more times, prior to another Delaney brother, Aaron, entering on 8 June and, as he looked around more thoroughly, he discovered Gabrielle’s body.
Lucas was then taken into the custody by NSW police officers at the hospital the following day, and he was subsequently charged with murder.
The defence of mental health impairment
Delaney’s defence lawyers arranged for forensic psychiatrist Dr Adam Martin to assess whether the defence of mental health impairment, contained in section 28 of the Mental Health and Cognitive Impairment Forensic Provisions Act (MHCIFP Act), was available for Lucas to argue in court. This defence can lead to a defendant being found “not criminally responsible for an offence”.
Dr Martin’s 3 December 2021 report on Delaney found that he’d used cannabis, synthetic cannabis and methamphetamine regularly in the past.
The report further outlined that at the time of the murder Lucas Delaney recalled being on the couch smoking cannabis with someone he’d met online, who, when asked to leave the flat refused, so he started swinging a tool at this man, which he could feel going through him as the man was laughing. Buit then on looking down, Delaney realised that the person had actually been his sister.
The report further outlined that Delaney claimed he didn’t know he’d killed his sister until the police arrived at the hospital to arrest him. The offender further gave evidence that he considered he was being pursued by the Freemasons and when he was in the hospital, he was continuing to hear voices, and everyone there had the same voice.
The psychiatrist further advised that while Delaney does suffer from schizophrenia, which can lead to delusions and thought disorder, he considered that arguing the section 28 mental health defence would be “contentious and problematic” because of his heavy drug use, especially the use of ice, which can lead a person to be “highly paranoid, agitated, angry and disinhibited”.
Section 4 of the MHCIFP Act defines a mental health disorder as ongoing disturbance of the mind that can be clinically diagnosed and can impair judgement. This can be in relation to anxiety, affective or psychotic disorders, as well as a substance-induced mental disorder that’s not temporary. But such a disorder does not include temporary impairment caused by taking drugs or substance use disorder.
The NSW Office of the Director Public Prosecutions provided a mental health report relating to Delaney produced by Associate Professor Anthony Samuels. In his report, he stated that he agreed with Dr Martin in that the defence of mental health impairment was not available to the defendant as the offence was triggered by the acute intoxication of cannabis and possibly methamphetamine.
Pleading guilty, as defence unavailable
Lucas pleaded guilty to the offence of murder of his sister in Penrith Local Court on 22 April 2022. He confirmed this plea to the Supreme Court of NSW on 3 June 2022.
The offence of murder is contained in section 18 of the Crimes Act 1900 (NSW), and it’s a crime that carries a maximum penalty of life imprisonment. The offence also carries a standard non-parole period (SNPP) of 20 years prison time, which means that an offender whose crime is assessed as mid-range of objective seriousness should receive a non-parole period of around 20 years.
Both maximum penalties and SNPPs are to act as guideposts for a sentencing judge to take into account, when considering the correct penalty to apply to a crime.
The plea of guilty to murder was entered after an initial March 2022 guilty plea to the lesser charge of manslaughter, the unintentional killing of another person, was not accepted. Section 23A of the MHCIFP Act provides that a person can’t be guilty of murder if they were substantially impaired due to a mental health disorder at time of offending, but this doesn’t apply to self-induced intoxication.
On 19 October 2022, NSW Supreme Court Justice Peter Garling then sentenced Lucas Delaney to 21 years imprisonment, with non-parole set at 15 years and 9 months gaol time.
Miscarriage due to faulty legal advice
Delaney then appealed his conviction to the NSW Court of Criminal Appeal (NSWCCA) on 14 May 2025, based on the one ground that asserted a miscarriage of justice had occurred because the applicant’s guilty plea to murder was entered on the basis of expert opinion and legal advice that was mistaken about the definition of mental health impairment in the MHCIFP Act.
The ground of the contention was that Dr Martin had not addressed the material question that is posed by the relevant legislation, “which is whether drug intoxication was the sole cause of mental impairment”, and this meant that the legal advice provided to Delaney was “necessarily and consequentially flawed” and the plea of guilt was entered on “mistaken advice”.
The legal advice ultimately given to Delaney provided that he did not have a mental health impairment, at the time of his offending, because he was instead intoxicated by illegal drugs. Neither of the experts who produced the assessments considered whether the substance use was the sole contributor to the crime, or whether it was reasonably possible his schizophrenia contributed to it.
Delaney’s lawyers on appeal added that if the experts had asked the question as to whether cannabis and methamphetamine use was the sole cause of the murder, then they might not have come to the conclusion that they did, as they would likely have considered his long-standing schizophrenia as being a contributor.
NSWCCA Justice David Davies pointed to section 4(3) of the MHCIFP Act, which outlines that a person does not have a mental health impairment if the person’s impairment was “caused solely by the temporary effect of ingesting a substance, or a substance use disorder”.
His Honour further explained that while the experts diagnosed Delaney as suffering schizophrenia, they went on to consider that drug use had cancelled out the potential for his mental health condition be a contributor to his crime, even though section 4(3) requires that the definition of mental health condition is only null and void when drug use is the sole cause behind a criminal act.
“As a result of being wrongly advised in relation to section 4 and its impact for the defence under section 28, there is a significant possibility that the wrong advice affected the outcome and the applicant has been deprived of the chance of a special verdict,” Justice Davies found.
“In that way, there has been a miscarriage of justice. In those circumstances, the conviction will be set aside and the matter remitted to the Supreme Court for trial,” he further ordered on 13 June this year.
And NSW Chief Judge at Common Law Ian Harrison and NSWCCA Justice Richard Weinstein both agreed with their colleague’s findings.
