Our changing climate is expected to see some plants release eight-times more pollen by 2100, worsening hayfever symptoms.

Allergies will become more prevalent and their symptoms worse, drinking water will be less reliable, and poisonous pests may find a new home in New Zealand's warmer climate.

These are just a few of the effects climate change is expected to have on human health, according to a newly-released Royal Society report.

Kiwis may have to contend with more intense heat waves and extreme weather events, including flooding and fires, which would take a toll on mental health.

SUPPLIED The invasive Australian redback spider has already appeared in Otago. But as our country gets warmer, it may spread to other places.

The Royal New Zealand College of General Practitioners said the stresses on children would compare to those growing up under the threat of nuclear war.

READ MORE:

* Wellington could become as hot as Sydney unless action is taken

* Some New Zealand climate change impacts may already be irreversible

* Editorial: There are some answers in a bleak climate change report

Royal Society president Richard Bedford said higher concentrations of CO2, increased temperatures and changes in rainfall were expected to extend growing seasons.

This would boost pollen counts and worsen hayfever symptoms. Some plants were expected to release eight-times more pollen by 2100, compared to in 2000.

Increases in other air particulates were also likely to increase asthma rates.

123RF Increases in certain air particulates as a result of climate change are likely to increase asthma rates.

Kiwis could also find themselves in the presence of some unwanted new arrivals from Australia and the Pacific Islands.

Harmful species, such as sea snakes and toxic jellyfish, may find new homes in New Zealand's warmer oceans. Drier inland areas may also become suitable for poisonous species, such as the invasive Australian redback spider.

Bedford predicted social disruption and the breakdown of communities, as people would be forced to relocate, either due to flooding and sea level rise, or because changed weather patterns made livelihoods unsustainable.

"If we think of the basic building blocks of health, such as our shelter, the air we breathe, water we drink and the food we eat, all will be affected by climate change."

Auckland University epidemiologist Alistair Woodward, a key author of the report, said the gastroenteritis contamination of Havelock North's drinking water in August last year was a wake up call.

The outbreak left roughly 5500 of the town's 14,000 residents ill and has been linked to three deaths.

ALDEN WILLIAMS/STUFF The stress on children of living with regular extreme weather events has been compared to growing up under the threat of nuclear war.

"It was a long dry spell and then the heaviest daily rainfall in more than 10 years. It really tested the limits of the health protection system," Woodward said.

"Christchurch got a whole heap of one-in-100-year floods at the start of the year, so that's going to become the pattern."

But the greatest impact on human health is likely to be increased droughts, with the eastern side of the country receiving less rainfall and the west receiving more.

SUPPLIED Royal Society President professor Richard Bedford says decision makers need more information on the health risks associated with climate change.

"The increase in drought frequency is going to put a lot of pressure on our rural economy," Woodward said.

"We know that there is a relationship between the rural economy, the welfare of the people working in the rural economy, and the frequency of mental health problems."

The report also predicts climate change will exacerbate existing socioeconomic and ethnic health inequalities.

SUPPLIED Professor Alistair Woodward, epidemiologist at the University of Auckland, says Havelock North's gastro outbreak was a wake-up call.

"You can think of it a bit as a threat-multiplier. Climate change is going to make life harder for people who are already suffering."

Children, the elderly, people with disabilities and chronic disease, and low-income groups were particularly vulnerable.

Worldwide diseases such as chikungunya and Zika virus would also likely make the jump from surrounding Pacific Islands.

MARTY SHARPE/STUFF Brookvale Road bore No.1, near Havelock North was one of the bores at the centre of an investigation into how the town's water supply became contaminated.

A spokeswoman for the Ministry of Health said district health boards were funded to detect exotic mosquitoes and ensure they did not become established.

DHBs were expected to include extreme weather event risks in their emergency plans, and a review had been commissioned of drinking water safety plans.

Incoming Minister for Climate Change Issues James Shaw said the report showed why climate change required an all-of-government response.

"Preparing for climate change and cutting climate pollution will require action in the health, agriculture, transport, energy and environment portfolios," he said.

"We plan to pass a Zero Carbon Act that will require the Government to create a plan every five years for addressing climate-related risks to New Zealanders. This will take into account risks to health as well as other areas."

* Comments on this article have been closed.