Wallabies are crossing the Waitaki River at the Aviemore Dam and colonising the south bank.
Wallabies, which are regarded as a serious animal pest, have traditionally been confined to an area in South Canterbury, centred in the Hunter Hills, but including the Two Thumb and Kirkleston ranges and the Grampian Mountains.
Environment Canterbury's (ECan) wallaby containment area is roughly bounded by the Rangitata, Waitaki and Tekapo rivers.
ECan biosecurity team leader Brent Glentworth said Ecan was aware of ''rogue colonies'' outside its containment area, such as in the Mackenzie Country and even in Ashley Forest, north of Christchurch.
He believed these had become established because people had illegally taken animals as pets and then dumped them or there had been a deliberate release by hunters.
However, it was likely the colony on the south side of the Waitaki had become established without human assistance. The animals had apparently migrated from the north across the Aviemore Dam.
ECan put a control programme in place when evidence was discovered at the site in 2008.
Since then more than 30 wallabies had been destroyed over an area of about 80,000ha.
ECan was working with farmers on both banks and Meridian Energy had plans to stop the spread by putting up netting as a screen behind the dam from Fishermen's Bend to Deep Creek.
Mr Glentworth said trend graphs created by counting faecal deposits had shown the population had been steadily increasing throughout South Canterbury. Because numbers had increased over the past decade they had ''in-filled'' a lot of areas.
Wallabies posed a significant threat to production and biodiversity, he said.
They had ''high dietary requirements'' and competed directly with stock for food.
Three wallabies were the equivalent of one stock unit (one sheep).
They competed with stock for pasture and crops, carried ''big worm burdens'' and their browsing prevented native bush regeneration.
Many farmers had been stepping up their level of control in response.
Mr Glentworth said a new tool in the wallaby eradication box was proving ''very effective''. When given wallaby Feratox (encapsulated cyanide), the animals died within one and a-half minutes of ingesting it.
This meant they died at the bait and its effectiveness was immediately apparent.