Up until the late 1950s, New Zealand houses used galvanised steel water supply pipes. Water and steel don’t mix well. Over time, although the exterior of the pipe looks okay, the inside of a pipe can start to corrode due to impurities in the flow of water. The corrosion restricts water flow, and eventually blocks the flow completely.
By the 1960s, plumbers moved to copper. It is very stable in most domestic situations, but raw copper piping eventually became expensive.
In the late 1970s, plastics were introduced. One manufacturer, Dux, imported a black polybutylene product known as Dux Qest (spelt without a U), and this quickly became the dominant piping product in domestic situations. A similar plastic was called Polytherm but was grey. This also suffered from the same issues.
This was imported and installed from 1977 to 1987 (with remaining stock used until the late ’80s), and many Nelson houses built or modified during this time have potentially leaky water pipes. You can identify Dux Qest because it is flexible black plastic piping with small white writing down one side, although it can be confused with another black plastic pipe that is actually polythene and used commonly in the garden.
Both Dux Qest and Polytherm have proved to be severely faulty.
Chlorine in the mains water supply reacts with this early formulation of polybutylene, causing the black plastic to erode from the inside out and become brittle. The plastic breaks down at high stress points, on bends or where the pipes are compressed by the crimp rings. This leads to pinhole leaks or sudden, full-blast splits inside wall cavities. Hundreds of litres of water can flood a home before the leak is noticed and the boundary mains turned off.
Since around 1990, a newer formulation of polybutylene that is resistant to chlorine has become the predominant material, with a few expensive homes still using copper.
Dealing with a leaking pipe inside a wall generally means removing wall linings in bathrooms and/or kitchens. Replacing these ageing wall linings and the old plumbing fittings can easily turn a pipe repair into a full $20,000 to $40,000+ renovation. It’s more difficult in two-storey houses and those with multiple bathrooms.
Most New Zealand home insurance policies heavily cap or exclude “gradual hidden water damage”, meaning the owner has to pay for the restoration.
During home renovations, plumbers often replace Dux Qest inside the bathroom being worked on, but can leave the hard-to-reach old pipework running through unrenovated wall cavities, under the floor, or out to garden taps. Because the pipes are behind plasterboard, a visual building inspection can only confirm what is visible in exposed areas (like the ceiling, subfloor, or hot water cylinder cupboard). You can’t guarantee that 100% of all pipework inside internal walls was removed.
This means that houses from this period, even if they have been renovated, carry a degree of risk.
It’s better to know about this issue before you buy a house and factor any costs into your purchase price.