In crystal clear drinking water, she drops to the bottom of the six mega litre tank and begins the cleaning process, slowly working the custom-built vacuum across the floor and inspecting the cleanliness and integrity of the tank.

It's a four-yearly ritual that doesn't usually see the light of day, but Queanbeyan City Council's 12 water reservoirs are now in tip-top condition after they were cleaned and serviced this week.

Contractor Aqualift Project Delivery travels across the eastern states of Australia performing this specialty task, and manager David Barry says there's a lot more to it than simply pulling on a dry suit and splashing around in a tank.

"It's really about giving the tanks a regular health check," he said.

"Nothing much happens in between us coming along, aside from the normal Council operations. So when we come and clean a tank we're looking for structural [issues], safety and security, water quality and just inspecting how that system works and comparing it with things we know.

"We're then giving the councils a report on the tank, a short term and long term shopping list so they can plan repairs and budget for the life of the tank," he said.

And if it sounds claustrophobic and creepy diving in the midst of a massive, dark water tank, the divers at Aqualift Project Delivery say it's a piece of cake compared to open water.

"The scary bit is diving out in the open ocean- you're in the middle of nowhere," Mr Barry said.

"Diving in a tank is like being in a building. You've got walls, you've got posts, you know exactly where you are.

"And there's no sharks."