Climate change is the likely cause of Northland's extreme weather woes, climate and weather researcher James Renwick says.
The Far North was shut off after heavy rain caused multiple slips, flooding and chaos as people were forced to leave their homes over the weekend.
Professor Renwick,the head of Victoria University of Wellington's school of geography, environment and earth sciences, said New Zealand should brace for more weather events like that.
He told RNZ's Afternoons it was fairly likely that the weekend event was due to climate change, although some analysis would be necessary to determine that.
"We see from research in most parts of the world that extreme rainfall events are on the increase, and the basic reason for that is the amount of moisture in the air - that's water vapour - increases as the temperature rises, a very straightforward relationship.
"So as it warms you get more moisture in the air, so when something happens to get that moisture out, like a storm, it causes a lot of the water to condense out, it forms rain, there's more moisture to start with so you are likely to get a heavier rainfall event."
Renwick said there were also very heavy rainfall events occurring now which had not been seen before.
Northland has just come out of a drought with Whangārei this year recording its driest January to April season on record, according to Niwa.
Renwick said the bad news about climate change was that it causes an increase in weather events at both ends of the scale - so as well as seeing more very heavy rainfall, it would also be causing an increase in droughts.
He said there was more moisture in the air generally.
"But when it's not stormy - you know, when we have sunny conditions, when the weather's settled - because it's warmer evaporation works better, works faster, soils dry out more quickly and we can end up with very dry conditions happening faster than they used to before."
"It's really unfortunate but this idea of having really dry conditions, having a drought and then have that followed by a really heavy rainfall event, is what we see for a lot of parts of the world in the future."
A motorist checks out the depth of a washout on Oromahoe Rd before opting to try a different route. Photo / Peter de Graaf
Renwick said flooding events were likely to increase in frequency and severity in New Zealand and elsewhere if greenhouse gas emissions were not reduced.
Temperatures in New Zealand have gone up by a little more than 1C on average in the last 100 years or so, he said.
He said it was difficult to track data on heavy rainfall events because often they were quite localised, so the data needed to consider a large period.
"But when you look globally, you do have to look over quite large regions to see this, but we do see very clearly that heavy rainfall events are on the increase around the world."
Renwick said the data also showed that bigger doses of rain were making up the annual total of rainfall which was bad news for farmers and gardeners.
