Asbestos debris has been found in a bush area near the Western Springs Stadium in Auckland, in what experts call a historic contamination unearthed by people making informal mountain bike tracks through the bush.
Warning signs have been put up at entry points to the affected area.
The asbestos material is broken pieces of fibrolite - or asbestos cement often used in buildings before 1985 - that has been there undisturbed for many years, says Auckland Unlimited, the city's economic and cultural agency.
A post on the Auckland Stadiums website says small amounts of asbestos were found in the bush next to a historic landfill near Old Mill Road and Bullock Track. The hazard area adjoins the Western Springs Outer fields.
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Auckland Unlimited said it is not possible to see where other asbestos pieces may be located due to the nature of the bush, or how much there is just under the surface.
All forms of asbestos can cause cancer. The risk from a single, low-level exposure to asbestos that has been bonded into fibrolite is extremely low, according to the Health Ministry, but no "safe" lower limit of exposure has been identified with certainty.
Auckland Council experts have visited the site to assess the risks to the public, and say the best way to deal with the situation is to leave the ground undisturbed.
People using the bush area should heed the signs and keep to the existing formed tracks.
Authorities have halted all replanting in the area and are looking to narrow down the contaminated area so bush restoration can continue in safe zones.