It feels like a ‘one-in-100-year’ weather event now happens every year. Flooding and landslides in the Nelson-Tasman region are the latest reminder of how vulnerable New Zealand is to extreme weather, and how we must prepare accordingly.
One of the most destructive costs is forestry slash: the woody debris of branches, treetops, and even whole trees left behind after logging. The impact is stark. Slash alters waterways, blocks fish passage, and suffocates estuaries. It can smoulder for weeks and even combust spontaneously.
In 2023, the Government responded to Cyclone Gabrielle and other slash disasters by proposing tighter forestry rules. Councils now have greater say over where commercial forests can be planted, and a national minimum standard for large-scale erosion-prone land has been introduced.
Yet local leaders argue these changes have been inconsistent, with stricter rules only arriving after the damage is done. Forestry groups warn that while regulation is necessary, increased compliance costs might impact profits. New solutions – such as converting slash into bioenergy – are in high demand.
When a major storm hits, slash can be swept into rivers and washed downstream, tearing through farmland and coastlines, snapping fences, damaging bridges and culverts, and piling up on beaches. Communities are left footing the bill for millions of dollars’ worth of clean-up and infrastructure repair.
The key to preventing this damage is to stop slash from moving off the land in the first place. Forestry operators are advised to keep streams and creeks as clear as possible, and to place debris where even a 20-year flood cannot reach. Slash traps, structures designed to intercept and trap slash in waterways to prevent their migration downstream, do help, but are like an ambulance at the bottom of the cliff.
Slash has to come from somewhere. Thousands of hectares of pine plantations were devastated in the Tasman floods, forcing the forestry industry to strategise. Heeg stated that landslides and a considerable amount of windthrow, trees toppled by the storm, were responsible for much of the forestry damage, rather than slash.
Nevertheless, communities downstream still had to face the damage. A bigger challenge emerges: whether forestry practices and land-use choices are well-suited to this era of extreme weather.
Flooding is the most frequent natural hazard in Aotearoa, with a major flood event occurring on average every eight months, says Environment Canterbury’s rivers manager, David Aires.
“Canterbury is home to many flood and hazard-prone areas and we’ve seen several significant flood events across the region in the last decade,” he says.
“With climate change effects, more intense and unpredictable weather events are likely to occur more often resulting in severe impacts on our communities. This highlights the need to invest in better flood resilience now to ensure essential lifeline infrastructure, such as roads and bridges, are safe in the future.”
Principal scientist of natural hazards for Earth Sciences New Zealand, Dr Emily Lane, adds that flooding is not only our nation’s most frequent hazard, but also one of our most costly.
“Treasury estimates that Cyclone Hale, the Auckland Anniversary Floods and Cyclone Gabrielle caused up to $14.5 billion of damage. While that was an exceptional year, on average, we could face around $200 million annually in damage to buildings alone. The recent events in the Nelson and Marlborough regions emphasise that flooding is a nationwide problem.”
Slash management is becoming not just an environmental issue but a life-saving necessity. “We need more investment in infrastructure to help reduce the impacts, like making sure we’re rebuilding with resilience in mind – stronger and in safer places when possible,” Dr Lauren Vinnell, Senior Lecturer of Emergency Management, Joint Centre for Disaster Research, Massey University, says.
The costs aren’t only financial. “Any experience of flooding can have harmful psychological impacts, but we know that it’s even worse when people go through multiple events, especially back-to-back, like we’re starting to see more often,” Vinnell says. “Unfortunately, it looks like repeated floods are only going to get more common, so we need to start preparing for this.”
So, what can be done? The Government has committed more than $10 million to research how woody biomass can be better used. Pilot projects are underway to test whether slash could replace coal in industrial boilers or feed into local bioenergy schemes. This aligns with the Forestry and Wood Processing Industry Transformation Plan, which aims to reduce waste, support biofuels, and extract more value from forestry by-products.
Bioenergy Association executive officer Brian Cox says we must not see slash as a headache, but as an energy solution. “While slash lies rotting in forests, it costs communities in terms of flood damage, but as fuel it could slice household electricity bills,” he says. With proper planning and pricing, biomass could provide up to 27% of New Zealand’s energy needs by 2050.
Energy from forest residue sells at about $16 a gigajoule (GJ), less than half the $36/GJ cost of electricity. A single gigajoule can heat 25–50 homes for a year; the average New Zealand household uses around 36 GJ annually. Cox says putting a price on slash would create incentives to clear debris before it becomes dangerous, turning a liability into a resource. The challenge in making bioenergy viable on a large scale lies in infrastructure.
In response to recent slash disasters, the Government is overhauling the forestry rules. Proposed updates to the National Environmental Standards for Commercial Forestry (NES-CF) include giving councils more say over where new forests can be planted, requiring large slash to be removed
(especially in erosion-prone zones), and mandating a Slash Mobilisation Risk Assessment (SMRA) as part of all harvest plans.
Yet critics say progress has been uneven. Local leaders in Tairāwhiti and Wairoa (and in parts of Hawke’s Bay) argue enforcement remains under-resourced and patchy. Forestry groups warn that while increased regulation is needed, it will bring higher costs and could drive some producers out of the industry, especially in regions with steep terrain or marginal land. Managing slash boils down to a tough balance between sustaining a major export industry while preventing social and environmental harm.
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