Nationals MPs want a dramatic shake-up of water laws to help drought-stricken farmers by taking water from the environment and selling it to irrigators in a move the former chief environmental water manager warns "fundamentally undermines" the goals of the Murray Darling Basin Plan.

The Nationals want federal laws changed as they scramble to respond to anger from constituents in regional seats and the threat posed by the Shooters and Fishers Party.

Nationals MP Damian Drum, Deputy Prime Minister Michael McCormack and One Nation senator Malcolm Roberts met with farmers who rallied on Tuesday to protest against the Murray Darling Basin Plan. Credit:Alex Ellinghausen

NSW Nationals Senator Perin Davey, a former irrigation consultant, is guiding the development of reforms that would set a trigger point to release from dams environmental water which was bought back from irrigators under the $13 billion Murray Darling Basin Plan.

The basin plan was initiated by the former Howard government to address the ailing health of the nation's biggest river system by investing Commonwealth funds to recover over-allocated water rights from farmers.