Waikato Hospital has had the last laugh after cutting off access to a sneaky tap installed by thieves in its pipeline of nitrous oxide.

Staff at Waikato District Health Board (DHB) discovered a spike in the usage of nitrous oxide - which is administered for pain relief but can also be recreationally inhaled as "laughing gas" - in May, with 10 to 12 weeks' worth being used in just three weeks.

The gas, worth thousands of dollars, is piped from large bottles under the hospital into a reticulation system and then into wards and departments.

Although the area under the hospital is locked and access is limited to contractors and a small number of DHB staff, staff discovered a tap had been installed in the supply line.

"Our security staff mounted a covert camera operation in consultation with the police as we suspected people were using the nitrous oxide for recreational purposes by tapping into the supply line with a pipe," the DHB said in a statement.

After the culprit failed to show up during 24 hours of surveillance, the DHB decided to shut down access to the area because of a risk to patients and those taking the gas.

It has fitted new security measures at all the potential access points in the supply line.

The DHB says there is no indication that the theft is continuing, and no further investigations are taking place, but it is appealing for anyone with information to come forward.