Port Macquarie MP Leslie Williams has walked away from The National Party.
Her resignation was made effectively immediately on September 20.
Mrs Williams, who has been with the party for nine years, has made an application to join the Liberal Party.
It is expected that application will be considered and finalised on Monday.
Mrs Williams said her decision was guided by the words she first spoke when she was sworn into NSW Parliament in 2011- that she "will continue to work hard, fighting for the best possible outcomes for the people in my electorate, who have given me the honour and the privilege to serve each and every one of them as the Member for Port Macquarie".
It comes after a tumultuous week for The Nationals after the party's leader John Barilaro backflipped on his threats to effectively bring down the coalition because of a rift over the state's koala protection policy.
Mr Barilaro said changes to the Koala Habitat Protection State Environmental Planning Policy were "wrong" and hurt landowners and farmers.
The Nationals leader said his MPs would abstain from voting on coalition bills and would be "effectively on the cross bench" as they fought changes to the protection plan.
Mrs Williams did not support this move. She described Mr Barilaro's behaviour as "politically reckless and unreasonable".
The defection provoked an ultimatum from Premier Gladys Berejiklian, who told Mr Barilaro and his Nationals colleagues to declare support for her government by 9am Friday or be sacked from cabinet.
At a crisis meeting on Friday morning (September 18), Mr Barilaro backed away from the threats. He is now on extended leave.
His deputy Paul Toole has been installed Acting Deputy Premier and state party leader.
Her electorate sits in Nationals heartland. Mrs Williams would not comment further on what that might mean for her politically into the future.
The seat has been held by the Nationals for all but six years since 1945.
"Last week, I advised the Deputy Premier that I would not be supporting his actions or those of my Nationals colleagues in effectively putting the entire party on the crossbench," Mrs Williams said.
"The NSW Liberals and Nationals have delivered unprecedented investment across the Port Macquarie electorate and to put this in jeopardy and hold the Premier and the Government to ransom during this COVID-19 pandemic was unnecessary, unhelpful and frankly politically reckless and unreasonable.
"The events of the past week have represented a further example of a course of conduct and dealing that has once again effectively been condoned and failed to be addressed.
"My community deserves to be represented in Parliament by members and leaders that are focused on their needs and their aspirations and they rightly expect to have policy overshadowing politics.
"Sadly, the events of the past weeks and months have cemented my decision to resign as a member of the NSW Nationals effective immediately."
Mrs Williams, a former nurse, was elected to the seat in 2011 after her third attempt. She became involved in politics in a bid to have a school built at Lake Cathie. That goal was achieved in 2014.
She has served in a variety of portfolios during her time including as Minister for Early Childhood Education, Minister for Aboriginal Affairs and Assistant Minister for Education.
She also held the role of Parliamentary Secretary for Renewable Energy and Parliamentary Secretary for Regional and Rural Health.
Mrs Williams is the Deputy Speaker of the Legislative Assembly.