Waikato District Health Board says about $5000 worth of nitrous oxide gas has been stolen from pipes beneath Waikato hospital during renovations.
An increase in the use of the gas, also known as laughing gas, was noticed last month.
The board says staff found a location on the nitrous oxide supply line where a tap had been installed and suspect it was being used to take the gas for recreational purposes.
Inhaling nitrous oxide in large quantities can be fatal.
Camera surveillance of the tap took place over a 24-hour period but staff were unable to work out who was stealing the gas.
The tap was subsequently shut off and security has been upgraded.