Scientists have confirmed a hidden spring well in the basement of Australia's diplomatic mission in London, thought to be at least 900 years old, is producing water good enough to drink.

The well is thought to be one of 20 throughout London that have been covered by roads and buildings over the centuries.

The spring under Australia House on The Strand is special because it is one of the few that are still accessible.

Australia House is already famous.

The building itself has been there almost 100 years, built during World War I.

The glorious marble interior appeared in the Harry Potter films as Gringotts, the wizards' bank run by goblins.

And it is understood Wonder Woman — the superhero movie set for release in 2017 — has also used Australia House as a backdrop.

But it is in the basement that so-called sacred waters flow.

High Commissioner Alexander Downer agreed to escort the ABC into the normally off-limits area to see the well and take water samples.

According to researcher David Furlong, the concept of the "holy well", or sacred healing spring, dates back to Celtic times and before.

Medieval monk wrote about well

Mr Downer said a monk from the medieval era wrote about the well water, describing it as sweet, wholesome and clear.

ABC cameraman Cameron Bauer and producer Donna Mountain at the well. ( ABC News: Lisa Millar )

"It comes from the Fleet, which is the river now covered by streets," he said.

"And you'll know the name Fleet Street which was named after it. It's now a subterranean creek."

In Roman and Anglo Saxon times the Fleet was a major river. But as London grew, it became a noxious, polluted ditch instead.

"These wells were of great significance, particularly back in the Middle Ages," Mr Downer said.

"They were used for ceremonial purposes and plays were performed around the well. And as a result of that this part of London evolved as an area where theatres were built."

There was a rusty can floating in the water the day the ABC visited to take samples.

"Someone has drunk the water and there's no record of them not surviving," Mr Downer said with a laugh.

Australian public servant Duncan Howitt admitted he was the one who drank a cup of it seven years ago.

He was encouraged by a colleague from the Canadian High Commission who had an interest in history.

"We all came down, there was about five of us, and as part of our hospitality for hosting them we drank a cup of the water," he said.

"It was fresh and clear. Better than tap water."

Laboratory tests declare water fit enough to drink

The ABC put Mr Howitt's taste buds to the test with laboratory Latis Scientific.

Scientist Richard Lawson tested for E. coli, Enterococci, Clostridium perfringens and total viable count.

The well water was tested at Latis Scientific laboratory. ( ABC News )

The results came back clear and he declared the water fit enough to drink.

Could he tell if it was holy or not?

"Well that's probably something you'd have to ask a priest and someone with religious expertise about," he said.

"We didn't have a vampire to throw it at."

The laboratory threw out the samples collected by the ABC, so Mr Downer did not have an opportunity to personally test the results.

He laughed at suggestions it could be bottled and sold as another money-earner for Australia House.