Tight financial management is expected to be needed at several district health boards where populations have fallen.
Statistics New Zealand has released 2013 census tables covering DHBs.
They show population decreases for Whanganui DHB, (3.4 percent); Gisborne-based Tairawhiti DHB, (1.8 percent); and Rotorua-based Lakes DHB, (0.1 percent).
But the largest DHB by population - Waitemata - is also the fastest-growing, at 9.1 percent with 525,555 people.
DHB funding is based on the census data, though this latest information won't flow into DHB funding decisions until 2015.
Other factors such as inflation and costs are taken into account when overall funding for each DHB is decided and DHB chiefs say this smoothing effect means none will suffer reduced funding as a result of the population changes.
General manager of services and business planning at Whanganui DHB, Tracey Schiebli, says it expects to receive a minimum increase to cover costs associated with high levels of social deprivation, an ageing population and disparities in health.
"The biggest challenge I see is actually the impact of the declining population on the economic status of the district. Because we know the link between economic status and wellbeing."
Tairawhiti DHB boss, Jim Green, says the population decrease came as a surprise, as Gisborne's population has grown.
He says Tairawhiti, which also has the largest proportion of Maori in its population of any DHB at 49 percent, will make up for lost population funding by other factors, such as more Maori and older people under its umbrella.
Mr Green says the population decrease came from rural areas and maintaining service levels in those areas will pose ongoing challenges.
"They are difficult areas to service anyway. And so reducing the populations in those areas just increases the difficulty of making sure that the people in those areas get the services they need."
Tairawhiti also has the highest proportion of regular smokers at almost 24 percent.
Chief executive of Waitemata DHB Dale Bramley says it's gained 60,000 people since the 2006 census and has had to invest $200 million into a building programme over the past three years to cater for the increase.
Dr Bramley says it's also made service improvements to cater for the rapidly growing population.
"We always need to be thinking, planning, looking to the future and being prepared for what's coming. So not only is the population growing but conditions within the population are changing."
Counties-Manukau DHB's population grew by 8.4 percent and Auckland DHB's by 7.8 percent and Statistics New Zealand's acting manager for census statistics, Victoria Treliving says that's in line with what it expected.
"So for example we saw growth at the Auckland region between 2006 and 2013 of about 8 percent, so it was consistent with what we saw there."
Health board populations in Wellington and Wairarapa grew by 6.4 percent and 6.5 percent respectively.
The Canterbury DHB district population rose 3.4 percent and South Canterbury DHB has the highest proportion of people aged 75 and over, at 9.3 percent.
Counties Manukau DHB had the largest percentage increase in that age group, at 23 percent.
