Megan Haines: I know how to commit the perfect murder

The former partner of a nurse who killed two elderly patients in a northern NSW care home has revealed how she boasted years earlier about how she knew how to 'commit the perfect murder'.

He helped put her behind bars after he read of a chillingly similar crime in the news.

"Yeah, yeah straight away I knew. By that time, yeah. I'd seen vindictive, I'd experienced it so I knew she knew no limits," Richard, whose identity has been protected, told Sunday Night.

Had the deaths of 82-year-old Marie Darragh and Isabella Spencer, 77, at NSW St Andrews Aged Care Home not occurred in the same night Megan Haines may never have been caught. They were both injected with fatal doses of insulin and appeared to have had massive strokes.

When Richard met Megan Haines in Melbourne in 2008 he didn’t realize that her disturbing theory of 'the perfect crime' was, in fact, a plan.

"She said, "I know how to commit the perfect murder."

"I said, "There is no such thing as a perfect murder. It's impossible." and she said "Yes there is."

"She said, 'Easy, just inject them with insulin' and I said, 'Why? Why insulin' and she said 'because when the body dies, it keeps assimilating the insulin and leaves no trace'."

The details she provided in that conversation would be replicated in her crimes years later.

"I said "Well what about the injection mark?'" Richard said, "and she said, 'Well if you're good they won't find it… Besides, it appears like natural causes."

Richard’s testimony provided crucial evidence for police as they built their damning case against Megan Haines.

Back in 2008, Haines was accused of injecting two elderly patients with insulin at Caulfield Medical Centre in Melbourne. They survived and Haines was never charged.

But on May 9 2014, her patients at Ballina's aged care home were not so fortunate.

Marie’s daughter, Jan, planned to come visit early the next day with a promise to bring pancakes.

"In the morning I received a phone call from one of the head nurses there telling me that they couldn't get any response from mum. They thought she'd had a massive stroke, and my husband and I left to go to the hospital straight away."

Two doors down the corridor, Isabella was already dead. Twenty minutes later, Marie also passed away.

Her granddaughter, Shannon, was by her side.

"I held her in my arms, and I rocked her, and I just told her "We're all here nanny. We all love you."

With suspicion surrounding the double death, police began tapping Haines' phone and sent off blood samples from both victims for testing.

Five days after the deaths, police came here to Megan’s home with a search warrant for drugs and she was told it was about the suspicious deaths of Marie and Isabella

The blood tests hadn’t returned and the cause of death was still unknown even by police.

It was then that, in a phone intercept, Megan revealed something only the killer would know.

"Um apparently some patients were actually given wrong medication," Megan can be heard saying in a call to the nursing home."

"To two people."

When the toxicology report arrived showing Marie and Isabella had been injected with a lethal dose of insulin, it was immediately clear to police what had happened.

Insulin is designed to lower blood sugar levels in diabetics. But in non-diabetics it lowers glucose levels to a point where the brain shuts down. What’s more, it quickly disappears from the bloodstream – meaning it’s all but undetectable.

Richard believes Megan may have used this method before and got away with it.

"You wonder how many times its happened before."

"If she’d done one lady Monday the other lady Friday it would be never ever anything done about it, she was home and free."

"I couldn't understand why she worked in the elderly sector when she had this attitude, and it was that the elderly were a complete waste of space and a burden on society and really they should be laid to rest earlier."

Haines was found guilty of murder in 2016.

"We got her and that's all that counts at this point in time," Marie's daughter Shannon said.

"She'll never be able to hurt anyone else again. That's one less monster [on] the street."