The NSW government will face another mid-term by-election test with Katrina Hodgkinson to quit parliament at the end of August.
The Cootamundra MP on Monday said she was resigning after almost 20 years in parliament and it was time to move on to something new.
"The decision is not one based on any single electorate issue, simply it is a 'length of service' decision," Ms Hodgkinson said in a statement.
The Nationals MP said she planned to spend more time with her family and "engage in opportunities that may arise within the corporate sector".
She will officially retire on August 31, her spokesman told AAP.
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Deputy premier and Nationals leader John Barilaro said Ms Hodgkinson had worked tirelessly for the people of southern NSW.
"She has contributed to public life for the better part of two decades and her experience will be missed by the parliamentary team," he said in a statement.
The outgoing Cootamundra MP was the first female minister for primary industries which Mr Barilaro described as "a remarkable achievement".
Ms Hodgkinson was overlooked for a ministry after the 2015 election and after Mr Barilaro took over from Troy Grant as Nationals leader in late 2016.
It's understood Ms Hodgkinson's decision has taken her party by surprise.
It was widely expected she would run again in her safe seat at the March 2019 election.
The NSW government has already faced by-elections in three seats this year.
In April, the Liberals retained the northern Sydney electorates of Manly and North Shore, while Paralympic gold medallist Liesl Tesch picked up the seat of Gosford for Labor.
