Helen Kelly, president of the CTU, has told delegates in an email that her diagnosis was confirmed by doctors this week, and she will be undergoing "lots of additional tests".
The extent and nature of the cancer was not yet known, she said.
"The hospital rang and they have confirmed I do have a lung cancer," she said in her email. "Nothing else is confirmed and I now have to have lots of additional tests and they have to test them etc etc. There is no confirmation of where else it might be or any prognosis.
"I have a string of appointments now and I hope by 9 March to have a plan of some sort."
Ms Kelly, who has been CTU president since 2007, will continue in her role, saying she felt "remarkably well, all things considered".
"I am writing this to you because rumours of my demise are exaggerated and I want to keep you all informed without the rumours," she said in her email.
The former teacher said she was "looking after myself", and thanked union delegates for their "beautiful support this week".
"It has been amazing."
Ms Kelly was not available for interview, the CTU said.
By Patrice Dougan of NZME. News Service
