Philip Nitschke of advocacy group Exit International said on Wednesday he was not on hand for the woman's death on September 23 involving the "Sarco" capsule in a forest in northern Switzerland but saw it live by video transmission.
The device worked as planned, he said, in the first and only time it has been used.
The head of a Swiss affiliate of Exit International known as The Last Resort, Florian Willet, was present at the woman's death and was immediately taken into police custody, where he remains.
Several other people who were initially taken into custody - including a journalist for Volkskrant newspaper in the Netherlands, where Nitschke lives - were later released.
The Australian-born Nitschke broke weeks of silence through an interview with Swiss newspaper Neue Zuercher Zeitung, which was published on Wednesday.
Speaking to the Associated Press by phone, he said he felt compelled to speak out because Exit International was "desperate" about the plight of Willet, who could remain behind bars for weeks or months until a possible trial.
The "Sarco," which Nitschke has said cost $US1 million ($A1.5 million) to develop and build, was designed to allow a person sitting in its reclining seat to push a button that injects nitrogen gas into the sealed chamber.
The person is then supposed to fall unconscious and die by suffocation in a few minutes.
The 64-year-old woman was not identified.
Nitschke, a trained medical doctor, said she had "compromised immune function" that made her "subject to chronic infection".
On October 26, Volkskrant reported the Swiss prosecutor had indicated in court that the woman may have been strangled.
"It is absurd because we've got film that the capsule wasn't opened. She got in herself, pressed the button herself - and Florian rang the police" after she died, Nitschke said.
Peter Sticher, the prosecutor for the northern Schaffhausen region who is leading the legal case, declined to comment in an email to the AP on Wednesday, citing an ongoing investigation.
Nitschke has repeatedly said Exit International's Swiss lawyers had advised that use of the capsule would be legal in Switzerland.
Swiss law allows assisted suicide so long as the person takes his or her life with no "external assistance" and those who help the person die do not do so for "any self-serving motive," according to a government website.
Switzerland is among the only countries in the world where foreigners can travel to legally end their lives.
Swiss police have confiscated the only operating Sarco device but Nitschke said another was being produced.
He said he wanted a "clear decision" from Swiss courts before using the device in Switzerland again.
Exit presented the "Sarco" to journalists over the northern hemisphere summer.
Before using it, the group had to overcome technical difficulties including its original designer walking out of the project, Nitschke has said.
