Landcare Research will be wrapping up research on the best size for possum poison baits in two forests near Wanganui this year, Dave Latham says.

He's leading the research project, which started in Southland in 2013.

In it staff put out single possum bait pellets of various sizes, some dyed green and poisoned with 1080. They set up trial cameras and put radio collars on possums taking the baits, to find out whether they survive or die.

At the moment poison baits used in New Zealand weigh 12g each. The experiment is to find out whether 9g baits would kill the largest possums.

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If smaller baits will work, costs can be saved and less 1080 will be applied per hectare, Mr Latham said.

The experiment has varied the seasons when baits are put out, to find out whether that makes a difference. And it has looked for places where baiting is not done often, to make sure the animals are not bait shy.

Single cyanide baits will also be put out, if necessary, to kill possums that survive the 9g poison baits - and make sure Landcare staff can retrieve the radio collars.

There will be about four staff putting out baits and watching how possums react around Trains Hut in the Waitotara Conservation Area in late June and early July. They do the same thing in the Mangapurua Valley from mid-August to early September.

Mr Latham said they have permission to work over a large area in each place, but will choose a convenient smaller area away from tracks and private land to lay the baits.

He has placed large warning notices in the Wanganui Chronicle and there will be others on site. The carcasses of poisoned possums will be highly hazardous for dogs for up to six months.

The trial will finish by December 20, at the latest, and the result will be published in a report next year.