Sydney - A staff member at an Australian nursing home was sentenced on Tuesday to 40 years in prison for murdering two elderly residents and attempting to murder a third with insulin injections.Garry Steven Davis, 29, was convicted in the New South Wales state Supreme Court in September of injecting Gwen Fowler, 83, Ryan Kelly, 80, Audrey Manuel, 91 at the SummitCare nursing home in Wallsend in October 2013. Manuel survived the injection, but died later of unrelated causes.Justice Robert Allan Hulme on Tuesday ordered Davis to serve at least 30 years of the 40-year sentence before he can be considered for parole.Hulme said there was no reasonable doubt that Davis injected each victim with an intention to kill, but that his motive remained a mystery."He maintained his innocence and exercised his right of silence in the trial and in the sentence proceedings," Hulme said."Accordingly, it remains a complete mystery ... why he carried out these abominable crimes and why he chose these particular victims," Hulme added.The offences were aggravated by the "extreme breach of trust" involved as well as the victims' vulnerability, the judge said. Davis had given thought to and prepared for his crimes, Hulme said.Davis's trial had heard evidence that he had sent text messages to colleagues in which he accurately predicted the deaths of his two murder victims.