Euthanasia activist Philip Nitschke says a Gold Coast mother and her two daughters had been members of his euthanasia group and started planning their deaths more than six months ago.
Margaret, Heather and Wynette Cummins' bodies were found in one of their homes following an apparent triple suicide pact on Tuesday.
Dr Nitschke told AAP on Wednesday he did not personally know the women but confirmed they had joined his Exit International group.
"It was a bit over six months ago," he said from Amsterdam.
"They were not terminally ill people but on the other hand, they were not in the best of health either.
"They were obviously working out what they wanted to do."
Dr Nitschke said three members had never ended their lives at the same time before, and it was also unusual all of them chose to use the same method.
But the man known as Dr Death said it appeared to be a "rational and informed choice" and their decision to end their lives because of their health problems was a matter only for them.
"It needs to be respected, it doesn't mean you need to be enthusiastic about it," he said.
The euthanasia advocate said Australia's laws were stuck in the dark ages.
Dr Nitschke said even though states like Victoria were considering voluntary assisted dying laws, they didn't go far enough because they only extended to terminally ill patients.
"People have all sorts of other reasons," he said.
"A rational decision by an informed adult needs to be respected and people should have access to the best information."
Dr Nitschke slammed Australia's refusal to allow his handbook to be sold.
He said in a statement on Wednesday the Cummins' family had subscribed to the online version of his book so they could research "a peaceful, reliable death".
A Gold Coast man returned to the luxury residential estate of Ephraim Island at Paradise Point on Tuesday to find his wife, mother-in-law and sister-in-law dead.
Detective Senior Sergeant Mark Procter told News Corp Australia the husband was shaken, but not completely surprised.
It's believed all three had medical problems and police aren't treating the deaths as suspicious.
Neighbours have expressed sadness over the deaths in the upmarket residential community.
Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.
Suicide Call Back Service 1300 659 467.