Fish & Game New Zealand has voted to phase out the use of lead ammunition in all shotguns used for waterfowl hunting.
Research has shown that the birds eat the fired lead-shot as a grit substitute for digestion and it poisons them.
New Zealand hunters with the larger 10 and 12-gauge shotguns have had to use non-toxic shot since 2004 if they are hunting waterfowl within 200m of open water.
Fish & Game spokesperson Don Rood said the council had decided to ban lead shot from the smaller 20-gauge shotguns.
By the 2021 shooting season, shotgun owners will have to use non-toxic shot when they're hunting ducks, swan and pukeko over water.
Mr Rood said this brought New Zealand in line with many overseas countries, including the US, Canada, Europe and some Australian states - where non-toxic shot for waterfowl hunting is already mandatory.
"The point that the New Zealand Fish and Game Council has taken is that it is going to require every one of its license holders to shoulder an equal part of this."
In the past it's been hard to find non-toxic shot for the smaller guns, but Mr Rood said Fish and Game had been talking to ammunition importers.
"To give them time to change their inventory, their ordering and source the products they need to be able to supply the users."
The move completes a process which began nearly two decades ago, and not all hunters were happy, Mr Rood said.
"There has been some resistance to it, mainly because it only applies to such a small number of hunters."
The only exemption would be for the small-bore .410 shotguns which are sometimes used by beginner hunters and for which humane non-toxic shot loads are not available.