Napier Library has been closed this afternoon after the discovery of picric acid, a potentially explosive substance, in a museum collection.

A NZ Defence Force explosive ordnance disposal team is checking the substance found in a collection in the basement below Napier Library. Photo: RNZ / Tom Kitchin

The library is in a building called the MTG, with a museum and theatre also on site.

MTG director Laura Vodanovich said staff were doing routine work in the museum collections about 12.45pm, in the basement below the library.

"The staff came across, in one of the medicine cabinets, a bottle with picric acid in it. Obviously, it's been there for some time but when we discover things like this, we take precautions to make sure everyone's kept safe."

They contacted the police and army, who advised them to evacuate the library, so no one was near the substance.

The library is expected to reopen tomorrow.

Police said a NZ Defence Force explosive ordnance disposal team was en-route from Wellington to assess the situation.

Vodanovich said it was not uncommon to find such items in museum collections.

"I guess because I've worked in museums for a very long time, you come across things, you come across asbestos, people have come across grenades or things like that and you kind of go 'is this live, is this not live'? You just check them out. It's only a potential risk, so we just take it seriously, we make sure we're acting prudently."

Picric acid is a pale yellow, odourless solid that has been used as a military explosive.

Friction or rapid heating can cause it to explode.

An empty Napier Library this afternoon after a potentially explosive substance was found. Photo: Tom Kitchin

A sign states Napier Library has been closed. Photo: Tom Kitchin