New Zealand is unlikely to see anything as hellish as the 50C days that scientists have just warned for Melbourne and Sydney over coming decades, a climate scientist says.

But that didn't make our country immune from the big swings that climate change would deliver, and our extreme events would come in the form of more frequent droughts, and heavier and more frequent extreme rainfall.

A new study led by the Australian National University painted a bleak picture for Australia under climate change, with unprecedented extreme heat and increased water temperature in the Coral Sea that would keep cooking the Great Barrier Reef.

Lead researcher Sophie Lewis says that even if the world limits warming to the 2-degree target set under the Paris climate change pact, Australia's biggest cities could still see heatwaves pushing the mercury to 50C.

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It will naturally be worse if that target isn't reached.

"Our current emissions trajectories for carbon dioxide and other greenhouse gasses in fact exceeds what can be expected for 2 degrees," Lewis told ABC television.

"We're currently tracking for more like 3 degrees, in which case extremes in Melbourne and Sydney and across Australia would be far more severe."

Researchers used extreme temperature records compiled by the Bureau of Meteorology, and then looked at how they might change under the Paris pact targets.

Those targets include an aspirational goal of limiting warming to 1.5 degrees - something many scientists now consider improbable if not impossible - and a more achievable target of below 2 degrees.

Lewis said those targets might not seem far apart, but the difference they'll make to Australia's liveability will be vast.

"At the moment the record temperatures in Melbourne and Sydney are much less than 50 degrees - more around 45, 46, 47 - and they're already a great challenge to people living in those big cities.

"If we're already challenged by 46 degrees we have to be thinking about how we can be prepared for 50, which will mean greater challenges in terms of emergency services, hospital admissions, human health and infrastructure."

The outlook for the Great Barrier Reef remained grim, after warming-driven coral bleaching events this year and last year.

But New Zealand's experience would be different, Victoria University climate scientist Professor James Renwick said.

Shifting the global mean by several degrees would have dramatic consequences anywhere, but the numbers were bigger in a place like Australia because of its geographic profile.

Renwick pointed out Australia was one of the warmest, driest, desert-covered continents on the planet - and much of it sat within the tropics.

"You get really hot over a big continent like that, whereas in New Zealand, we don't see that effect as we are surrounded by ocean, and that does keep the temperatures down and stops them varying that much.

"So New Zealand is not going to have 50C days - it would have to get a hell of a lot warmer for that to happen."

Renwick expected the closest New Zealand analogue to that would be Christchurch, which could see more days in the 40s.

According to Niwa, the mid-range estimate for projected New Zealand temperature change is for an expected increase of about 0.8C by 2040, 1.4C by 2090, and 1.6C by 2110, relative to the 1986 to 2005 period.

Owing to the different possible pathways for the concentrations of greenhouse gases in the atmosphere, however, as well as the differences in climate model response to those pathways, the possible projections for future warming span a wide range - between 0.2C and 1.7C by 2040, 0.1C to 4.6C by 2090, and 0.3 to 5.0C by 2110.

Projected changes in rainfall showed a marked seasonality and variability across regions.

It was very likely that for winter and spring there will be an increase in rainfall for the west of both the North and South Islands, with drier conditions in the east and north.

Extreme rainfall was likely to increase in most areas, with the largest increases being seen in areas where mean rainfall is also increasing, such as the West Coast, while drought severity is projected to increase in most areas of the country, except for Taranaki-Manawatu, West Coast and Southland.

- Additional reporting: AAP