THE 104-year-old Australian scientist who has chosen to end his life at a Swiss clinic says he would prefer to have done so in Australia.

"Australia is behind Switzerland in this move, as are most countries," Professor David Goodall said at a final press conference in the Swiss city of Basel on Wednesday.

He was accompanied by Dr Philip Nitschke, founder of Australian right-to-die group Exit International, and Swiss clinic Life Circle's lawyer Moritz Gall.

"At my age, or less than my age, one wants to be free to choose the death when the death is at an appropriate time," Prof Goodall said.

"My abilities have been in decline over the past year or two, my eyesight over the past six years. I no longer want to continue life. I'm happy to have the chance tomorrow to end it."

Prof Goodall, who was born in 1914, flew from his home in Western Australia, where his daughter, son and grandchildren also live, to France last week to see relatives before arriving in Switzerland.

"Luckily my family who are in various parts of Europe and America have rallied round and come to see me, and I welcome the opportunity to see them, which I probably wouldn't have had if I hadn't pursued this Swiss option."

He spent his last day touring the Basel University Botanical Gardens with three of his grandchildren.

"I feel very privileged that I will be able to be there when my grandfather passes away," Mr Goodall's 30-year-old grandson Daniel told the Daily Mail.

"He is so brave and I am so glad that he has been able to make his own choice.

"It is his wish that he can end his life, but such a shame that he was not allowed to do it in his own country," he said.

Another of the botanist's grandchildren, Duncan, 36, also told the publication he would be by his grandfather's side when he passes away.

"I think what he is doing is incredibly brave. My grandfather has approached this in a completely rational way and not let any emotion get in the way.

"He wants to die and he wants to die on his own terms. The fact that he is doing this so publicly shows how brave he is," he said.

The professor was also filmed cheerily singing a few bars of Beethoven's Ninth Symphony.

Prof Goodall was cleared by two Swiss doctors, including a psychiatrist on Wednesday who said he was of sound mind, to proceed with the "Swiss option".

Australian scientist David Goodall says goodbye to his grandson at Perth Airport. Picture: AAP Image/Sophie Moore
Australian scientist David Goodall says goodbye to his grandson at Perth Airport. Picture: AAP Image/Sophie Moore

The esteemed botanist and ecologist, who does not believe in the afterlife and has been a member of Exit International for 20 years, said he tried clumsily to take own his life at least three times, and then finally decided to get professional help.

He hopes his story will help countries like Australia change their laws to be more accepting of assisted suicide.

"I certainly hope my story will increase the pressure for people to have more liberal view on the subject," he said.

"I think there probably will be a step in the right direction."

Victoria was the first state to pass a euthanasia bill last November but it doesn't become legal until the middle of 2019.

Prof Goodall will end his life at 10am Thursday, local time, in the presence of family members.