"The Wairarapa has a mostly rural population spread over a huge geographical area, which bucked the expected trend and rose in numbers in the latest census," said Dr Milne.
"The make-up shows a growing trend of older people.
"Conversely, Hutt Valley's mostly urban population has not grown as quickly, with a rising number of younger people."
Following a strategic meeting last week and the decision of the joint chief executive to step down, the board of Wairarapa DHB agreed to recruit a separate chief executive.
"We have our own unique health challenges and want to increase our own local decision-making but remain committed to building on the collaborative gains of the last three years."
Dr Virginia Hope, chairwoman of Hutt Valley and Capital & Coast DHBs, said: "We expect the three DHBs in the region to continue working closely together to improve the health of our communities."
Both chairpeople said their boards' policies of working collaboratively with neighbouring DHBs, through a number of arrangements which shared clinical, management, planning and executive functions, had improved patient experiences, quality of care and productivity in the region.
