"Fermented grain, fishing line, a rock and a cage," says ACT Parks and Conservation Service manager Brett McNamara.

"You don't want too many moving parts."

The formula may be simple, but it's effective.

More than 275 feral pigs have been removed from Namadgi National Park this year as part of the ACT government's annual cull, up from the usual number of about 200.

The feral pig control program has been extended this year, with the government working with 20 rural landholders in southern parts of the ACT to roll out and place pig traps.

In collaboration with the National Landcare Program, the government is also targeting pigs in the Murrumbidgee River Corridor, Molonglo River Reserve and areas of Canberra Nature Park.

Fermented grain is placed at the back of a large cage to attract the pigs, with the door held open by fishing line weighted down with a rock.

When the pigs move the rock to get to the grain, the door slams shut and rangers switch the bait for a lethal poison, only once they're sure no other animals are inside the cage.

Mr McNamara said pigs usually travelled in family groups, and up to six could fit in one of the cages.

"A feral, female pig, over a 12-month period, can have up to two litters of around half-a-dozen feral pigs," he said.

"So when you think about the total number of pigs being removed, there is also that multiplier effect.

"Pigs are very similar to rabbits in that if they're left unchecked, they can run rampant right across this environment."

Mr McNamara described feral pigs as a "scourge on the environment" and said they could have a significant impact on sensitive areas including the water catchments within Namadgi National Park, which are the main source of Canberra's drinking water.

"Not only do they affect the integrity of the water catchments, they spread weeds, they affect us in terms of disease control, and they also have a negative impact on agricultural land."

Andrew Geikie, who owns 3000 acres of land in Tharwa, said the extension of the feral pig control program had been a "great initiative".

Mr Geikie, who received new pig traps as part of the National Landcare Program, said feral pigs had grown in numbers in the past 10 years, moving more and more out of timbered areas and onto farmland.

"As far as our bottom line goes, they eat a lot of lambs and we'd be losing, we estimate, 30-40 lambs a year," he said.

"Our neighbours have taken a bigger hit, further up towards Tidbinbilla, with lamb losses in the vicinity of 50-100 per cent.

"Financially, that’s in the area of $50,000-plus damage by the pigs."

Mr Geikie and fellow Tharwa farmer Mathew Gregory said they usually caught about 30 or 40 pigs on their properties each year.

Mr Gregory expected that to increase to about 60 on his farm this year after receiving one of the new traps.

He said his main issue with the feral pigs was that they dug up pasture and created erosion, which he said cost him about $5000 a year.

He welcomed the chance for landowners to work more with the ACT government on feral pigs.

"Over the years, the pigs have made their way down from the mountains and established themselves on our area," he said.

"We’ve got together now as a coordinated team to have a go at them."