The woman who had a nitrous oxide canister signed by Simon Bridges says the National politician was "blissfully unaware" of what he was putting his name on to.

An image of the canister, known colloquially as a nang, appeared on social media on Thursday with the leader of the opposition's signature emblazoned across it.

The canister contains a colourless gas commonly used for sedation and pain relief.

In recent years, they have become popular to some as a form of altering the state of mind if the nitrous oxide is inhaled to produce a short-lived, dissociative high.