"I like the river and I think with our walkways and parks it's a really high-profile place to start," he said.
The area could be enlarged, suburb by suburb, to cover the whole city. The group was just at a "baby stage" now.
"I want to get everybody in the room and get it going."
A likely starter is Milly Mitchell-Anyon, who lives by the river and sees rats climb into the rubbish bins there. She was planning to start her own group, and asked the Department of Conservation.
Mitchell-Anyon was a member of Predator Free Mt Cook for a year, while living in the Wellington suburb of Newtown. She joined the group by email, logged into a system, got a trap and recorded her kills online.
The group had about 900 traps across a small area, and had killed 3034 predators - mostly rats, mice and hedgehogs and a few possums and stoats, she said.
Mitchell-Anyon owns three traps already.
She said dead animals could be "a bit gross", but she could shake a dead rat out of the tunnels without touching it.
Predator Free Whanganui was the only Whanganui group that had applied to the Predator Free New Zealand Trust for funding, general manager Jessi Morgan said.
Its next funding round is in about six months.
However, there is a Predator Free Marton group on Facebook, and the Quaker Settlement and Tutaenui Stream Restoration Society do predator trapping.
Morgan is keen to see more happening, and so is Horizons Regional Council biodiversity, biosecurity and partnerships manager Rod Smillie. A predator free New Zealand was an aspirational goal, he said, and "you need an aspiration if you want to get there".
Horizons doesn't do predator trapping right across its region - only in particular places such as Pūkaha Mt Bruce and the Manawatū Estuary.
Making it general would increase the region's rates burden, and that would be unpopular with some, he said.
He was heartened to see community groups taking on the trapping themselves.
"I'm hopeful. I think people do want to see our biodiversity flourish," he said.
• To find out more about Predator Free Whanganui, email Chris Smith at predatorfreewhanganui@gmail.com.