"Indirectly, I think the staff numbers are around 15,000," Simpson said.
"My position is to ensure the accountabilities for delivery of healthcare for the entire region are met, offer advice on the strategic needs of the region, and to identify and mitigate key risks and escalate them nationally."
Health New Zealand replaces the 20 existing district health boards and will be in charge of the country's health system both clinically and financially.
It will own and operate services, set the framework for commissioning, and arrange the provision of services at a national, regional and local level.
A smooth transition from DHBs to Health New Zealand, including the continuation of planned care, needed to be ensured in the coming weeks, Simpson said.
The central region, one of four across the country, covers around 954,000 people.
"Another part of this new role is looking at our winter preparedness and continuing our Covid-19 response.
"Financial performance is also a key element, along with overseeing capital projects for the central region."
Simpson said the two major capital projects in Whanganui were the hospital's chemotherapy and transfusion unit and the development of the Ruapehu Wellness Centre.
Under the Health New Zealand structure, people could be moved around the central region based on areas of demand and capacity, Simpson said.
"I see that as a significant strength to a local health service provision when you can leverage off the capability across the region, rather than rely on the individual district to service the entire population on their own."
The entire health system had been under significant pressure for a number of years, and it had been exacerbated by Covid-19, Simpson said.
"Having said that, there is a lot of opportunity in the new Health New Zealand format, with the Māori Health Authority, to really work collectively, more regionally, and in a way that will benefit the entire population rather than an individual district."
On the whole, the district health board system had served the country really well, allowing a local voice in the provision of health service delivery, Simpson said.
There had been "variable interactions" between DHBs, many of which were positive.
"They key challenge though is each of the district health boards managed their budget and access to services based on their individual district.
"Now those boundaries are gone."
Simpson said it was his understanding that the Māori Health Authority's regions would mirror those of Health New Zealand.
There were mixed emotions about leaving his current role.
He is already transitioning into the Health NZ position.
"I've thoroughly enjoyed my four-and-a-half years as chief executive of the Whanganui District Health Board, particularly moving through what I've seen as one of the most challenging times the health system has faced.
"At this stage I still plan to base myself out of Whanganui, but I will be in Wellington pretty much every week, and I'll commute to Palmerston North and Hawke's Bay."
