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Mr Cull (70) was diagnosed with a tumour on his pancreas earlier this month, and he knew at that stage he was likely to face a tough journey ahead.
His chemotherapy treatment started this week.
Mr Cull had three terms as mayor and decided not to seek re-election last year, opting instead to pursue a position on the Southern District Health Board.
He was elected and then appointed chairman in December 2019, but his stint with the board lasted only until October 15 this year, because of the health challenge he faced.
Mr Cull said the effects from chemotherapy varied between people.
"I don’t expect it to be an easy ride - no-one does."
He felt some fatigue, which he understood could be caused by the treatment, or by the cancer itself.
Mr Cull described the treatment process as being "injected with a whole lot of bloody poisons, really" and taking pills to offset the side-effects.
He said he continued to be uplifted by messages of support.
"I appreciate the support so many people have offered."
Deputy chairman David Perez has taken over as acting chairman of the SDHB board.