Victoria Police is investigating after a gas bottle was deliberately emptied into a busy hotel overnight, north east of Melbourne.

Investigators said tragedy was narrowly avoided after around 150 patrons fled the Commercial Hotel in Alexandra after noticing the smell.

The Commercial Hotel in Alexandra. (9NEWS)

One person was treated by paramedics for gas inhalation.It is believed the 45kg gas bottle was deliberately emptied into the Grant Street venue after midnight.

The venue was busier than usual with the Alexandra Truck, Ute and Rod Show being held over the long weekend.

"We need to catch the person responsible so they can be made aware of the potential ramifications of their actions," Detective Leading Senior Constable Peter Clifford said.

Anyone with information about the incident is urged to contact Crime Stoppers on 1800 333 000.