A witness said the incident happened near the toilets about 3pm. The man had burns to his leg and said the vape in his pocket had exploded.
A St John Ambulance spokesman said a man was taken to Dunedin Hospital in a serious condition.
Four fire crews were called to reports of a small explosion and smoke inside Reading Cinema in the Octagon about 2.55pm.
When they arrived they found a small battery had exploded and there was no fire, a Fire and Emergency New Zealand spokeswoman said.
Although uncommon, vape fires and explosions can occur. The US FDA said in a statement in 2019 that it was "concerned" about "overheating and exploding batteries".
It recommended that consumers consider "using devices with safety features, preventing loose batteries from contact with metal objects, using the correct charger and not charging [a] battery overnight or [leaving] it charging unattended."
