Australia should consider having a one-child policy to protect the planet, an environmental lobby group says.

Sustainable Population Australia says slashing the world's population is the only way to avoid "environmental suicide".

National president Sandra Kanck wants Australia's population of almost 22 million reduced to seven million to tackle climate change.

And restricting each couple to one baby, as China does, is "one way of assisting to reduce the population".

"It's something we need to throw into the mix," the former Democrats parliamentarian told AAP.