A south Otago farmer thought he was doing the right thing when he dropped off some old explosives for disposal at the Balclutha police station yesterday morning.

But miscommunication with the police led to part of the town being evacuated and the army called in to detonate the unstable explosives.

The farmer found the 65 sticks of gelignite and detonators while he was clearing out his late father's property, Sergeant Martin Bull of Balclutha police said.

"He rang last week and spoke to someone here at the police station and was told to come and see us. But I think there was a bit of miscommunication as to what he actually had," he told NZPA.

The farmer brought the explosives to the Renfrew St police station about 9.30am.

"We had a look at the items and they were quite unstable. Normally gelignite is quite stable, but if it does get old it starts sweating or crystallising. Then it becomes unstable, which is what happened with this stuff."

Police evacuated the station and nearby businesses. Several streets were cordoned off, and an army explosives team flew from Christchurch to help dispose of the explosives.

Five hours later, after police were called from Invercargill along with the fire service and St Johns, the army blew up the gelignite at a local quarry.

Police had spoken with the farmer and no charges would be laid.

Mr Bull said the Defence Force was regularly called out to deal with old explosives.

"Apparently a lot of that work comes from the southern district, so it looks like there's quite a few people who've got leftover explosives from years and years ago. It does seem to be prevalent for the area."

Mr Bull urged people to get in touch they wanted to get rid of old explosives.

"If people do have some leftover explosives from either their parents or for other reasons, give us a phone call and we'll assist in getting rid of them," he said.

"Especially if you've got really old stuff there, rather than move it round, just leave it and give us a phone call, and we'll sort it out."

Mr Bull said some people might be afraid to come forward, but they had "nothing to fear at all" in approaching police.

"Obviously if there's legitimate reasons for having it, then everything's fine. If they've come into possession of the items illegally, then we would be concerned."