David Seymour says once the End of Life Choice Act is voted in, it will be difficult to shake.
Preliminary results of the cannabis and euthanasia referendums will be revealed at 2pm this afternoon.
The Act party leader told Mike Hosking it's taken five years to prepare, along with 16 months in the select committee process.
He says if the result is what he hopes, the law will be in place for a long time.
"Members of parliament generally run a mile from these type of issues. If the result is what we hope for today, we will have this law for a long time."