Contractors were at the receiving end of an explosion in Christchurch's CBD this afternoon.

Emergency Services were called to the Ara Institute of Canterbury at around 3.30pm.

Four people received minor injuries, all were treated at the scene.

An Ara spokesperson says contractors inadvertently ignited vapours generated from contact adhesive.

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The area is a work site and was already closed off to staff and students.

They say there was no gas bottle involved.