Sir Michael Cullen has resigned from his role as chair of the Bay of Plenty Health Board and as a member of the Lakes District Health Boards due to stage four lung cancer.
He said the news was "totally unexpected".
"What was initially a CT scan of my heart has resulted in a clear diagnosis of Stage IV small cell lung cancer with multiple secondaries in my liver.
"Chemotherapy is likely to extend my lifespan somewhat, but it is clear to me I will not be in a fit state to carry on all that I have been doing in recent months."
He had taken on the roles after serving as Finance Minister and Deputy Prime Minister under Helen Clark.
"I took on these roles knowing that they would involve a heavy workload but offered a real opportunity to make a difference in our health system in the Bay of Plenty.
"I have already stood down reluctantly from my long-held position as lead claims negotiator for Te Kotahitanga o Ngati Tuwharetoa.
"The only major role I will maintain in the meantime is as Chair of the Earthquake Commission (EQC) to see it through the release and the response to Dame Sylvia Cartwright's report into EQC and the Christchurch earthquake sequence. I expect to make a decision shortly about the timing of my departure from that position."
Both district health boards have expressed their sadness and wish Sir Michael, his wife Anne and their family well.