Former state and federal Labor MP Kelvin Thomson has joined the Sustainable Australia Party as an adviser.
The former federal Wills MP will advise the party's first MP Clifford Hayes, elected to Victoria's upper house at the November state poll.
"There are irreconcilable differences between the course I believe Australia and the world needs to chart, and the course that the Australian Labor Party is charting," Mr Thomson said on Monday.
Mr Thomson served as ALP councillor from 1981 to 1988, Victorian Labor MP from 1988 to 1996, and Federal Labor MP for more than 20 years, until the 2016 election.
He said after representing the Labor party for 35 years, his resignation came with a "very heavy heart".
"For a decade now I have set out what I believe to be the myopia, greed, vanity and ecological illiteracy that drives Big Australia, Australia's policy of rapid population growth," he said.
"I retain a hope that in time, the Labor Party will embrace views about Australia's population that are more in keeping with the needs of this generation, the needs of those who will come after us, and the needs of the many other species we have the good fortune to share this ancient, beautiful and fragile country with."
Sustainable Australia Party founder and president William Bourke said Mr Thomson played a key role in kick-starting the population debate a decade ago, describing Treasury projections of a 35 million population by 2049 as a recipe for environmental disaster.
Mr Hayes said Mr Thomson's knowledge of government and his campaign and policy experience, having successfully stood for public office 12 times, made him an invaluable asset.
Australian Associated Press