In a letter to the chairman of the authority, Craig Knowles, on July 28, the NSW Office of Water Commissioner David Harriss asked for a 560 gigalitre rise in the amount of groundwater allowed to be taken from six key aquifers, citing the needs of mining companies.

Friends of the Earth, which obtained Mr Harriss's letter through freedom of information, said it was proof the government was putting mining interests ahead of the environment.

Last month it was revealed the authority is indeed set to recommend a large rise in the amount of groundwater allowed to be taken from right across the basin, although it now denies this was in response to the urging of NSW.

Mr Harriss wrote: ''These groundwater systems are brackish, undeveloped and not connected to surface water, and represent the only potential source of water for future mining requirements in these areas.

''The revised volumes are considered sufficient to meet likely demands for the mining industry for the period of the Basin Plan (2012 to 2022) while ensuring that extractions remain well within sustainable limits.''

Friends of the Earth Murray-Darling campaigner Jonathan La Nauze said the NSW government had been ''caught red-handed'' quietly paving the way for the mining industry to get large quantities of groundwater.

''It is very disappointing to see that the MDBA has capitulated to NSW,'' he said. ''These particular groundwater systems are very poorly understood and we simply do not know enough about them to be massively increasing water extraction without causing major damage.''

The revelations came as the federal Environment Minister, Tony Burke, urged state governments to toughen conditions for approving mining projects that could affect groundwater.

''One of the things we really need to look at here, because of the importance of water underground, is to make sure that the states come up to the same standards for underground water that we've put in place at a federal level,'' Mr Burke told the ABC's Insiders program.