The soggy question posed by New Zealand floods

This week, northern Canterbury and southern Marlborough experienced the worst flooding in possibly more than a generation. Fuelled by a deep, broad, slow moving low that delivered huge rainfall tallies in the northern South Island – and significant snow in inland and southern areas – many rivers that in summer can be forded on foot – made bold bids for freedom. And the mess left behind poses a seriously soggy question to resource management planners about the cost of repairing rural infrastructure.

The flooding is likely to have caused possibly tens of millions of dollars in damage, mainly on farms but also to rural infrastructure and the State Highway network. A number of rivers broke through their stopbanks and recorded some of their highest ever peak flood discharge – on the Conway River there is a 10% annual exceedance probability for an 800m3/s flood. The one that went down on Tuesday afternoon peaked at 1,537m3/s at State Highway 1. The Mason River, which is usually fordable on foot was surging at 664m3/s. In the time that the modern State Highway network has existed in northern Canterbury, there probably has not been such a sustained flood assault on the roads.

Flooding is the most frequent natural hazard to occur in New Zealand, and it is not difficult to see why when one considers the distribution of built-up areas in relation to rivers and their flood plains. It is in many cases the result of development encroaching on active riverbed, that has flooded since European settlement began. Councils in their determination to keep development happening in their cities/districts/regions have sometimes made trade offs that they have come to regret.

A good example is the development of residential zoned land around the Ashley River in central Canterbury. The river flooded badly in 1953 and flowed across land on its floodplain that now hosts Rangiora, one of the fastest growing towns in rural New Zealand. Its riverbed has been significantly restricted, and choke points where the river becomes particularly narrow have not always worked as intended.

This weeks flooding poses questions of another kind. Many rural districts in Canterbury – and around New Zealand – have small ratepayer bases, which expect perhaps unrealistically, their elected councils to fix all infrastructure that gets damaged. Some of the roads damaged have been little used routes, perhaps only by farming vehicles, trucks carrying gravel or other aggregate for roading and construction purposes.

Who pays? Do the ratepayers fork out for the repairs? Do certain parts become user pays – and so, how would this be determined and implemented?

But it is not just local councils that have to address this problem. Right now, State Highway 1 in the Leader River valley, is closed because the river washed away a significant chunk of the portion of the road that runs alongside it. This is not a local council job, despite what a large number of people on social media seem to think. Management of the State Highway system belongs to New Zealand Transport Authority. Thus, the money spent fixing the significant damage caused by the Leader River and other rivers during the storms earlier this week will come from a national budget.

It is not just roading networks that should be concerned by the implications of the flooding that has just happened. The Kowhai River, which goes out to sea a few kilometres south of Kaikoura, broke out for the second time in less than 35 years when it started flowing across farmland towards the township on Tuesday afternoon. If we used a 3% annual exceedance probability as a starting point, it could mean someone who is 40 and has lived in Kaikoura their whole life, could conceivably see a third Kowhai River break out before they die – 24/12/1993, 07/07/26…

As climate change – which I believe is happening – takes hold, more intense storms may make this otherwise quite extraordinary event more mainstream. While it is true that Kaikoura catchments can see 300mm of rain in a day, this only happens every several years. To have nearly 400mm fall in 30 hours, is thus a quite significant event whose annual exceedance probability is not something many people, including planners, take into account. Rivers that have been strangled by development are drawing very soggy lines in the proverbial sand.

No doubt all of this will be on the minds of Environment Canterbury river management, Kaikoura District Council and N.Z.T.A. officials as they contemplate their maintenance priorities for the financial year. It will pose questions for the planners going through the 3 Year and Annual Plan cycles about the allocation of council rates, and whether certain roads, properties that seem to be flood prone are now liabilities on the budget spreadsheet.

And as the winter of 2026 is still comparatively young, what is there to say that there will not be another heavy rainfall event in north Canterbury/Marlborough before the years end?