Sir Bill English has warned against making euthanasia legal in New Zealand, saying descriptions of the End of Life Choice Bill as a "slippery slope" don't go far enough.

The former Prime Minister says the Bill, initially tabled by ACT leader David Seymour, will make New Zealand a "dangerous place".

"This is a serious issue that could change the nature of our country," he told The Project on Monday.

"This law will make New Zealand a dangerous place for people who are young and suicidal or old and think they're a burden, or people with disabilities.

"If they have a bad day, where they wish they weren't alive, we should be offering them kindness - but what we're going to offer is to kill them."

But Seymour says while it's easy to "speculate and fearmonger", data shows countries haven't seen a rise in youth suicide or back-flipped on their decision to legalise euthanasia. He says the law change won't put anyone at unnecessary risk.

"What this End of Life Choice Bill will do is put in a regime of safeguards... that will be sufficiently safe and give better protection to people."

But Sir Bill says any law change on euthanasia sets a dangerous precedent that could give rise to worrying developments - particularly in regards to mental health.

"It's worse than a slippery slope… once you start saying 'we'll drop the ban on killing, some people can choose to be killed' - how do you stop more and more and more?"

Watch the full video from The Project.