Australia's oldest scientist is planning to fly to Switzerland in early May to end his life, despite not having a terminal illness and reigniting a national euthanasia debate.

David Goodall, who is 104, believes his quality of life has deteriorated and he has secured a fast-track appointment with an assisted dying agency in Basel, according to euthanasia advocates.

"I greatly regret having reached that age," the ecologist told broadcaster ABC on his birthday earlier this month. "I'm not happy. I want to die. It's not sad particularly. What is sad is if one is prevented.

"My feeling is that an old person like myself should have full citizenship rights including the right of assisted suicide," he added.

Prof Goodall made headlines in 2016 when his university tried to force him from his role on the grounds that, at the age of 102, he was no longer fit to be on campus.