A former detective has been brought in to crack down on "cowboy" plumbers but a gaping hole remains around dodgy plumbing products.

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The Plumbers, Gasfitters and Drainlayers Board is ramping up its clampdown on who does the work, adding a second investigator next week alongside the ex-policeman and introducing a new "report-a-cowboy" app.

But it has no remit to investigate products and there are claims too much is just too dodgy.

Temuka plumber Murray Bartlett said poor workmanship and bad products often went hand in hand.

"I had one the other week where they supplied a cheap shower mixer and I said to them, 'I'm sorry I can't install that' - but an unauthorised tradesman wouldn't do that.

"If I decide to import some stuff from India or China and I flick it off on Trade Me, nobody asks the question 'does it comply with the New Zealand standard?'"

Mr Bartlett said he believed a flood of poor product being imported unchecked was a factor behind stubbornly high rates of building inspection failures in Auckland.

A third of the city's 130,000 or so inspections failed in the last year, though the Auckland Council talks mainly about poor workmanship by a rump of less competent builders than about dodgy building products.

Some MBIE investigations under way

Poor workmanship also bugs Milton Sands, a director at big Auckland pipe merchant Aquatherm.

But he also estimated half of all plumbing products being used here had not been certified to the New Zealand standard, though many would still be good products.

What it showed up, though, was the lack of controls.

"Until we have some certifying body in New Zealand checking products, the end user is always at risk. It's got to be [central] government; local government hasn't got the resources."

It's manifestly unclear just how many of the thousands of taps, cisterns and pipes for sale are a failure waiting to happen.

The Ministry of Business, Innovation and Employment has the power to warn about or ban building products - but it hasn't ever done so.

It said the "presence of this regulatory tool provides a threat that modifies behaviour and incentivises voluntary product withdrawal or change".

The ministry did now have a couple of product investigations under way and those would help test its efforts to improve on its surveillance and intervention, it said.

It was also planning a major campaign this year to, it said, "promote" the use of building products that complied.

Word of mouth

Firm figures to judge the problem are hard to come by but anecdotes abound.

Plumbers, Gasfitters and Drainlayers Board chair Peter Jackson worked as a plumber in Invercargill before retiring to Queenstown.

He said Chinese manufacturers had offered to put the brand name of his choice on tapware.

Mr Sands, from Aquatherm, said his firm and others created pressure over a plumbing pipe product that was cracking and it was withdrawn.

The Department of Building and Housing did not respond to their concerns, he added.

The cracking pipe, marketed as Dismee, was being sold as being certified under Australia's Watermark compliance system.

"But when we actually checked up on that product, the [certification] number actually belonged to an obscure light fitting."

Similarly, Mr Bartlett said when a Timaru council inspector checked on a suspect drainage pipe he found the certification did not stand up.

"The drainlayer who installed it had to pull it out. We all knew what was happening - we all just figured it isn't worth the candle to buy that stuff so we will just stick to the New Zealand-made product."

So, as it stands, word of mouth remains the main protection against dodgy plumbing products.