NSW Labor's deputy leader Prue Car is taking leave to undergo treatment for cancer, saying the diagnosis is a shock but her prognosis is "looking positive".
"Following recent tests, I was found to have a large tumour on my kidney," the Londonderry MP said in a statement on Monday.
"This has come as quite a shock."
The 39-year-old-year-old deputy to Chris Minns broke the news to her western Sydney constituents on her Facebook page saying: "Friends, I have some hard news to share."
"Prognosis is looking positive but I will be taking some leave for cancer treatment over the coming weeks."
Mr Minns said the entire Labor Party was behind her and he had "no doubt at all that she will fight this with her customary flair and courage".
"You're talking about one of the most determined, toughest people in politics today," he said.
"She will bounce back quickly. We can't wait to have her back on the frontlines working with us for the people of NSW."
The opposition spokeswoman for education and early childhood acknowledged the "hard working, compassionate, skilled but overworked doctors, nurses and allied health professionals who are guiding me through this process".
Ms Car was a Penrith City councillor and national communications manager at MS Australia when elected as an MP in 2015.
She was previously an advisor to former Labor premier Bob Carr from 2003 to 2005 and campaign co-ordinator of the Labor Party from 2005 to 2007.