A faulty piece of PVC pipe has forced Canberra's Captain Cook Memorial Jet water display on Lake Burley Griffin to be switched off indefinitely.

Authorities decided to turn off the water jet after major flooding was discovered in its extensive underground pump house.

Helen Badger from the National Capital Authority said the damage was caused by a broken PVC collar.

"We couldn't have predicted this failure," Ms Badger said.

"There was significant flooding in the pump house due to the failure of this fitting," she said.

"We needed to be safe, firstly, and then dry so that we can get down into the space and investigate the problem and determine the extent of the damage."

The jet was first switched on by Queen Elizabeth II on the April 25, 1970 and its main pump shoots water at 260 kilometres per hour, sending six tonnes of water up to 147 metres in the air.

The pump house is a four-storey structure hidden beneath Regatta Point.

"It's quite an impressive structure under the ground and extends down a couple of levels... so it's not just your average everyday pump," she said.

"There's a lot of electrical equipment down there, so we needed to get everything isolated, which we've now done as of this morning.

"Now we've got fans and temporary pumps in there to remove the water and more importantly dry everything out."

Ms Badger said until the extent of damage is determined, it was difficult to say when the jet will operate again.

"I don't think it'll be years... but I can't say that obviously," she said.

"We've got an impressive team of people from the National Capital Authority, as well as our contractors who are doing everything they can.

"I'm hopeful it'll be shorter rather than longer term."

The jet received regular maintenance but had experienced occasional problems over the decades.

Most recently, it was turned off last year during a blue green algae bloom on the lake.