The scientist then decided it was time for him to die, Dr Nitschke said.

"I no longer find much joy in life. Up to, even up to, say, the age of 90 I was enjoying life. But not now," Prof Goodall told the ABC.

Goodall worked at Perth's Edith Cowan University, and said he had become resentful about having to go abroad to die due to the absence of voluntary euthanasia laws in Australia.

There were a few tears but most of the friends and family who gathered to farewell the 104-year-old at Perth Airport on Wednesday said they had prepared themselves to say goodbye.

From his wheelchair, he hugged them all in turn, thanked them and wished them a wonderful life before he and his nurse boarded the plane.

Doctors who initially treated him had threatened to prevent him from leaving Australia but he ultimately left the country unhindered.

A crowdfunding effort from Exit International, a euthanasia advocacy group, raised almost $US20,000 to cover Goodall's travel costs, including two business class tickets for him and his nurse.

Prof Goodall is currently being assisted by an organisation in Basel and Dr Nitschke.

He flew to France on Wednesday to visit family, with the procedure planned to take place in Switzerland on May 10.