He added: "I greatly regret having reached that age".
After his death, Mr Goodall's friend were asked to call the police and report an extraordinary death - a requirement of Swiss law.
"In fact his last words were: 'This is taking an awfully long time!'"
Since age 102, Goodall's health and quality of life deteriorated, and he wished to end his days peacefully and on his own terms.
The scientist flew business class from his home in Perth to Switzerland, where assisted suicide is legal, after raising thousands of pounds in donations to fund the journey.
David Goodall ended his life in Switzerland today.
Dr Goodall told reporters on Wednesday: "One should be free to choose the death, when death is at an appropriate time".
The state of Victoria in Australia recently passed a euthanasia bill, the Washington Post reported. The laws differ for euthanasia, in which a doctor actively administers a lethal drug, than for assisted suicide, which require the participants to take a type of active measure, like taking pills themselves. Even with the best palliative care, many people still experience unbearable physical and mental suffering as they approach death.
"It would have been much more convenient for everyone if I had been able to, but unfortunately it failed", he said of the suicide attempt.
Renowned ecologist David Goodall had travelled to the assisted suicide clinic from his home in Australia, where euthanasia is forbidden.
Still academically active in his later years, Goodall became an honorary research associate at Edith Cowan University, where he continued to review, edit and publish papers, as well as co-supervise botany students.
Yesterday, he told a packed news conference: "There are many things I would like to do, but it's too late".
"We must be very careful with life", Basel City Councilwoman Annemarie Pfeifer said.
"He was an awesome scientist, an fantastic man who did whatever he said he was going to do", she said.
Dr Goodall will reportedly die at 10am local time (9am United Kingdom time) at a clinic in an undisclosed location, with four family members and a close friend by his side. "In Switzerland, I will also see one or two other members of my family, and so that will also be a goodbye". However, the professor said that he hoped the large mediatization of his case would lead to the legalization of assisted dying in other countries.