Megan Haines, 46, had been working at St Andrews Village at Ballina for two months when she allegedly injected unscheduled insulin dosages to Isobella Spencer, 77, and Marie Darragh, 82, during the night of May 10 or early on May 11.

Ms Spencer and Ms Darragh died shortly after they were found unconscious by other nurses on May 11.

Police allege the surviving victim, 88, survived because she woke as Ms Haines tried to inject her with an unscheduled and unnecessary dose.

Police said the injections to the arms were the only injuries the patients suffered and it was not known if the victims were diabetic or required insulin.

Detective Superintendent Michael Willing of the Homicide Squad said police would allege Ms Haines injected the women after they lodged complaints against her.

Ms Haines will be charged with two counts of murder and one of assault shortly after she arrives in Sydney late on Tuesday night.

Detective Superintendent Willing said Ms Haines had been a suspect since early in the investigation.

“We had a good idea of what occurred and were waiting for further forensic accounts,” he said.

According to police, Ms Haines did not return to work after the deaths. She resigned a few days later.

St Andrews Village chief executive Phillip Carter told Fairfax Media he was unable to comment about how Ms Haines became aware of the complaints or its hiring procedures.

“Our sympathies are with the women’s families. Both women were very much part of our community, our family at St Andrews, and we share in their families’ grief,” Mr Carter said in a statement.

Ms Haines’s registration as a nurse includes a note the she is required to submit performance reviews from her managers to the registry office every six months, for 18 months.

A spokeswoman for the Australian Health Practitioner Regulation Agency, which manages registrations, declined to say specifically why the note had been added to Ms Haines’ record.

She said there were many reasons why such a note may be added, ranging from needing additional training to poor performance or behaviour.

Ms Haines was arrested on Monday in the Victorian coastal town of Seaspray about 11am. Warrants were issued for her arrest by the NSW Homicide Squad late last week.

She was remanded into Victorian police custody overnight before NSW Police successfully applied for her extradition at a court hearing on Tuesday morning.

During her extradition hearing in Victoria, her lawyer Joanne Finlay said her client required medication for severe depression and was vulnerable as this was the first time she had been taken into police custody.

Ms Haines has not requested bail and will face court again on Wednesday morning.