The impact of slash is more significant than many realize, as it can cause serious damage to roads, bridges, property, and other vital structures.
Understanding this threat is crucial to safeguarding our infrastructure and ensuring the safety of our communities.
Slash refers to forestry debris, such as branches, wood chips, and logs, that is left on the land after timber harvesting.
During Cyclone Gabrielle in 2023, the washout of woody debris and sediment destroyed a bridge in the Gisborne Region of New Zealand. This devastation also impacted farms, properties, and infrastructure along its path.
The destruction prompted a government investigation, which uncovered inadequate regulations and practices in the forestry sector. The investigation provided recommendations to prevent similar incidents in the future.
Forestry and wood products are New Zealand’s fourth-largest export sector, contributing $5.89 billion to annual export revenue, which represents 1.3% of the country’s GDP.
As of April 2024, radiata pine forests cover 1.79 million hectares in New Zealand (Forestry, 2025). While the forestry sector is crucial to the country’s economy, there is a significant need to improve slash management to prevent it from becoming a destructive hazard.
In New Zealand’s East Coast and Gisborne District alone, they produce around half a million tons of slash per year, representing up to 10% of the harvested timber, which is mainly considered waste.
An article from Forest360 notes that the country produces around 30 million tonnes of timber per year. Applying the average 10% ratio means that the country is producing around 3 million tonnes of slash annually.
According to an article in Interest, the government is spending $10 million to remove 70,000 tons of slash, which comes to $145 per ton. The cost of slash removal is slightly higher than that of harvested timber, at $130 per ton.
However, 70,000 tonnes of slash is just a drop in the ocean compared to the total the country produces, and current government subsidies make the cost seem unsustainable.
With the sheer amount of slash New Zealand produces at roughly 3 million tons per year, could there be another industry or another opportunity to turn this resource into something useful or into another product?
Slash for Cash is an initiative by The Tolaga Bay Heritage Charitable Trust, located in Gisborne. It aims to transform wood debris and other forms of dead carbon matter into organic biochar fertilizer and smokeless charcoal briquettes in the aftermath of Cyclone Gabrielle. Additionally, the program provides employment opportunities for local residents.
Slash for Cash uses a mobile carbonization kiln to harvest and process slash from the region, producing brick-shaped organic biochar fertilizers.
These fertilizers have a variety of applications, including restoring eroded land, filling patches on mountains, planting forestry trees, coastal restoration, de-silting soil, and enhancing the growth of crops, vineyards, and gardens.
They can also help with flood control and provide protection against future floods. These carbon bricks can last up to 30 years in the soil.
Their briquettes made from slash also serve as a sustainable, clean energy source for cooking, barbecuing, heating water, or heating homes for people staying off-grid.
Thabiso Mashaba of Slash for Cash tells The Post that their services can be replicated across the country and even globally to manage slash, while providing a sustainable energy solution and restoring the environment.
They aim to use 4320 tonnes of slash annually to produce 180 tonnes and 240 tonnes of biochar at each of their three sites, and to sequester up to 12,960 tonnes of CO2 annually, starting in 2027.
Another initiative that turns slash into a profitable and sustainable product is the Pārengarenga Incorporation (PINC), based in Te Kao, a village in Northland.
PINC aims to convert 17,000 tons of slash into biochar by smoldering woody offcuts in large kilns. The process used to make biochar is an ancient one that employs pyrolysis—heating organic material in a low-oxygen environment to create a solid, highly carbonized substance known as biochar.
This material has numerous applications; when applied to soil, it can help retain moisture and improve overall soil health. In New Zealand, using biochar on farm pastures can significantly enhance their quality, benefiting farmers.
The process of converting slash into biochar is currently labor-intensive; however, increasing the scale of the operation will make it more feasible. Each year, 7,000 tons of slash can be transformed into 1,600 tons of biochar, and New Zealand has an abundance of slash available for this purpose.
New Zealand’s experience with forestry slash highlights both the environmental risks of unmanaged waste and the promising opportunities for sustainable innovation.
By supporting initiatives that transform slash into valuable products such as biochar, the country can reduce hazards, improve land management, and create another profitable industry.
With government support and subsidies, start-up companies and entrepreneurs can establish a profitable, sustainable forest industry in New Zealand that transforms forestry byproducts into a successful circular-economy model.
Sources
Mathias, S. (2023, May 16). Forestry slash devastated Tairāwhiti and Wairoa after Cyclone Gabrielle. The recommended changes are drastic. The Spinoff. Retrieved from https://thespinoff.co.nz/society/16-05-2023/forestry-slash-devastated-tairawhiti-and-wairoa-after-cyclone-gabrielle-the-recommended-changes-are-drastic
Trafford, G. (2023, October 9). Guy Trafford takes a look at what to do with forestry slash, including a Fiji example and a possible role for the Climate Emergency Response Fund. Interest.co.nz. Retrieved from https://www.interest.co.nz/rural-news/124654/guy-trafford-takes-look-what-do-forestry-slash-including-fiji-example-and
Musson, M. (2025, August). August 2025 Market Update. Forest360. Retrieved from https://forest360.nz/august-2025-market-update/
Forestry and wood processing data. (2025, December 18). Ministry for Primary Industries. Retrieved from https://www.mpi.govt.nz/forestry/forest-industry-and-workforce/forestry-wood-processing-data
Shaw, A. (2026, January 5). Profit & Planet: Turning slash into carbon-trapping bricks. The Post. Retrieved from https://www.thepost.co.nz/business/360886973/profit-planet-turning-slash-carbon-trapping-bricks
Round, S. (2025, November 30). Country Life: Turning forestry slash into something good for the planet. RNZ. Retrieved from https://www.rnz.co.nz/news/country/535239/country-life-turning-forestry-slash-into-something-good-for-the-planet
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