The mortar was found near the public toilet, part of the Department of Conservation historic reserve at the ghost town. There was no information about whether it was found on top of the ground or discovered through 'fossicking'.
West Coast police area prevention manager senior sergeant Vicki Walker said the shell was found by a couple of children in "a reasonably public area" at the site.
Police dealt with the find in a routine way by isolating the mortar site to within 100m of it and contacting the army to deal with it, Walker said.
DoC was unable to shed any light on the incident.
West Coast partnerships spokeswoman Jose Watson said: "It's interesting because we have guys up there all the time looking after the place, so you would think it would show up."
She noted there had been reports of vandalism at Waiuta in the past few days, which staff were checking.
- Greymouth Star