The NSW government was warned by its own bureaucrats more than five years ago that reliable water supply in parts of the state could plunge by a third, with the situation now so dire that some of the largest regional towns face being left with no water within the next year.

The warnings were contained in a draft paper, Assuring Future Urban Water Security, which was dated December 2013 and commissioned by the Department of Primary Industries.

The cover of the Department of Primary Industries' 2013 draft report, Assuring Future Urban Water Security.

However, further modelling and studies were called off and the report was left incomplete.

The draft modelling in the report, conducted by NSW Public Works, considered the possible impact of climate change on dam levels and water supply at 11 regional councils, and found the amount of water that could reliably be made available in those areas would fall by between 9 and 30 per cent by 2030.