When two people, armed with a hammer and knife, attempted to rob a Rotorua dairy the owner set off a fog cannon, sending them running.

About 8pm last night two people entered the Pleasant Heights Dairy on Thomas Cres.

Rotorua police area prevention manager Inspector Brendon Keenan said the people were armed with a hammer and a knife and had demanded cigarettes and money.

He said the dairy owner then set off a fog cannon, which sent the pair running.

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A fog cannon is a security device that fills rooms with dense fog in a matter of seconds.

Last year, the Government set aside $1.8 million to help at-risk businesses who could benefit from these devices.

They cost up to $5000 to install but once in, could save a lot more than that by preventing theft.

Keenan said once the cannon was activated the would-be thieves became disoriented and ran from the store.

"Nothing was taken and nobody was injured, which is a success."

What is a fog cannon?
- A security device that fills rooms with dense fog in a matter of seconds. It is activated by an emergency button.
- It is made of a combination of glycol and water forced through a heated element. It is used in smoke machines and smells of peppermint.
- The fog is harmless to humans, animals, fixtures and IT gear.
- It clears in about 20 minutes in a well-ventilated room.