The trio had fallen ill after attending a family lunch in the Victorian town of Leongatha in July. A fourth person was taken to hospital in a critical condition.

The woman who served the lunch, Erin Patterson, was taken into custody on Thursday. She has not been charged.

Ms Patterson, 49, has strongly maintained she is innocent.

Victoria Police said the woman would be interviewed by detectives and a search warrant had been conducted at her house.

Gail and Don Patterson - the parents of Ms Patterson's ex-husband - were guests at the lunch along with Gail Patterson's sister Heather Wilkinson and her brother-in-law Ian Wilkinson.

The four were taken to hospital on 30 July reporting violent illness, police say.

Within days the Patterson couple, both 70, and Ms Wilkinson, 66, had died. Mr Wilkinson, 68, later recovered.

Erin Patterson has continued to reiterate her innocence, saying she herself was taken to hospital after the meal due to stomach pains.

She has said she served a beef wellington pie using a mixture of button mushrooms bought from a supermarket, and dried mushrooms purchased at an Asian grocery months earlier.

By Hannah Ritchie
BBC News, Sydney