ALMOST a third of the 863 billion litres of water that Prime Minister Julia Gillard and Water Minister Penny Wong have claimed will be returned to the rivers of the Murray-Darling Basin does not exist, according to official figures.

At least 270 billion litres that the government claims to have bought as part of the $1.4 billion spent so far will never assist a stressed river or sustain a wetland plant because of the type of water rights purchased, a federal Department of Environment, Water, Heritage and the Arts analysis shows.

It is what water irrigators call ''phantom water'', ''virtual water'' or ''airspace in the dam''.

A statement by Ms Gillard and Senator Wong posted on the ALP website on Tuesday said: ''Since the government started buy-backs in February 2008, federal Labor has bought back one out of every 20 litres of the Murray-Darling Basin's irrigation water and returned it to the rivers - some 900 billion litres of water entitlement.

''This is nearly five times Adelaide's yearly water supply.''