THE Tillegra Dam is being built for drought conditions predicted to occur only once in every 10 million months, or 830,000 years, internal planning documents have revealed.

But Hunter Water Corporation says the consequences of running out of water are so dire that it must act to ensure it never happens, no matter how minute the likelihood.

Justification for the controversial 450-billion-litre dam is underpinned by a scenario where a one-in-a-100-year drought - similar to 1979-80 - occurred four times back-to-back, documents uncovered in an upper house call for papers show.

The estimated probability of experiencing such an event ''is around 1 in 10,000,000'', Hunter Water said in response to questions raised by an independent review of the dam's justifications by SMEC Australia.

In comparison, Sydney Water's drought security criterion is based on one in 100,000.