Dr Philip Nitschke administered barbiturates at the man's home in the presence of his wife. The man, in his 60s, had advanced prostate cancer.
"I was the treating doctor, and the patient asked whether I would be able to help him end his life," Dr Nitschke said. "This is the first time ever a man has legally ended his life. There was a sense of history."
Doctor-assisted suicide became legal in the Northern Territory on 1 July after its parliament passed a voluntary euthanasia law last year.
In an open last letter before his death, the patient defended his right to end his life and criticized church groups and others wanting to quash the law.
"The church and the law should be separate," he said. "If you disagree with voluntary euthanasia don't use it, but please don't deny the right to me to use it if I want to."
Dr Nitschke, who has long fought for the right to help patients end their suffering, described the death as a "poignant moment" that had been planned for some time.
"On the day elected by the patient, he communicated to me that he wished to go ahead, and I was able to carry out that particular wish of the patient. And things went ahead very smoothly, very well, and quite appropriately," he said.
The man had been hooked up to a computerised medical device that released a lethal dose.
When Dr Nitschke unveiled the machine last year, he said a patient could indicate a final wish to die by pressing a key on bedside laptop computer. Alternatively a patient could change his or her mind at the last moment.
The patient, who died on Sunday, had met strict conditions to use the law, which requires a terminally ill person to be evaluated by two doctors and a psychiatrist, and a nine-day waiting period before a lethal injection can be administered.
The law is now being challenged in court by anti-euthanasia doctors, churches and some aboriginal groups. It might also be overridden by other legislation being considered by Australia's national parliament.
Opponents of euthanasia condemned Dr Nitschke. "I feel ashamed to be an Australian," said Margarate Tighe of the Right-to-Life group.
Until now doctors, who had feared they could be charged with murder if the law eventually was struck down had refused to use it.
Other patients who had travelled to Northern Territory for euthanasia have died of natural causes, unable to get other doctors to help them.
Dr Nitschke said the doctors who approved the death had felt remorseful about those cases and this time decided to help. He said he had thought deeply before agreeing to perform euthanasia and he had no regrets.
"It was an important task. I felt proud and privileged to help out this person who was in deep suffering," he said adding that the man had "died with dignity".
In the United States, a federal court blocked voluntary euthanasia legislation in Oregon.In the Netherlands, euthanasia is technically illegal though doctors may help patients commit suicide under certain conditions.Reuse content