The Otago Regional Council’s environmental implementation committee was told breeding populations were believed to be "extremely low" in the region.
But there had been more than 100 sightings of the animals over the past four years.
"Although the sightings inspected are considered genuine, it is very difficult to find evidence of wallaby in most cases," a report to the committee said.
"This indicates that public sightings are picking up transient individuals that are passing through an area and leaving little or no trace, unlike breeding populations that can leave abundant evidence (i.e. prints, scat, tunnels in scrub and tussock) in one place."
Cr Gary Kelliher said he believed the nature of the sightings across the region, often in isolated locations, suggested some animals were being released after outgrowing would-be pet owners’ ability to care for the animals.
"I still firmly believe that the issue for a lot of these scattered sightings are pet releases, because there are hunters who pick up fawns, they pick up piglets, they pick up kids, goats, and they also pick up joeys."
Cr Kelliher told the committee this week he had offered to be a confidential point of contact for those who found themselves with a rapidly growing wild animal at home, but council staff had dismissed the idea.
"I do not see people, who suddenly find that they’ve got a pet joey that’s grown up into a great big wallaby sitting on the couch, are going to be ringing the ORC or ringing MPI (the Ministry for Primary Industries) for advice. But I think the ORC given the effort and cost that it takes to go after a sighting, I think my offer, which was largely pooh-poohed by staff when I made it, should be back on the table."
Cr Kevin Malcolm said the animals’ expanding range over the years was worrying for Otago.
The threat they posed was "quite sensational".
On a visit to South Canterbury, a Waimate farmer told him he had shot 2500 wallabies this year, Cr Malcolm said.
"The Grampian Station (in the Mackenzie District), if I go back 18 months, the last time I talked to them, in the three previous years, they had shot 10,000.
"At the moment, we have not got those numbers, but we are seeing those animals come into the area.
"The spread of them is quite scary; they are devastating on the landscape."
Tracking of the animals showed one individual covered 42km in one night, he said.
And the council had covered more than 1 million hectares searching for them.
Farmers he had spoken to were supportive of the council’s work, "but we are running this on the smell of a non-oily rag".
The joint council-Tipu Mātoro National Wallaby Eradication Programme budget of $850,000 contrasted with the $1.4 million per year that national programmes project delivery specialist Gavin Udy had suggested, which was "probably a very realistic figure", Cr Malcolm said.
Council chief executive Richard Saunders said there were ongoing conversations about the national wallaby programme, "and we can continue to advocate for better understanding of the cost-benefit at a national level".
In terms of where central government invested, he said the council would advocate for better data to be used in the decision-making.
