Laboratory tests have found taps with 70 percent more lead leaching into drinking water than what is allowed.
The Master Plumbers industry group says the tap is just one among many unsafe products on the shelves and the government must act urgently.
The market for plumbing products is largely unregulated and unpoliced, though alarms have rung about lead leaching from products for many years, leading to a standard being set in 2005 of a maximum 10 micrograms of lead per litre of drinking water.
One tap tested at 17 micrograms and a second of the same type at 14 micrograms in the lab tests on five types of tap, commissioned by Master Plumbers.
"It's really bad for New Zealand consumers," Master Plumbers chief executive Greg Wallace said.
"The World Health Organisation deems that no level of any lead is safe, and obviously this is coming through the drinking water for New Zealanders."
His group bought the $180 mid-range kitchen mixer on Trade Me - where it was labelled only as ''FTL Tap 002, Number Code: 2" (the other four tap types passed the testing).
Trade Me would not say who the importer was, citing the Privacy Act; it has removed the online listing. Trade Me said the seller no longer sold taps on its site; that it had alerted two buyers; and had told officials that five taps in total had been sold through its site.
"The reality is that that importer would have brought in a range of taps, and to be fair to the reseller, currently the standard is only voluntary so they're actually not breaching any current law," Mr Wallace said.
"It's not just tapware, there's other products ... that are leaching lead into the water system."
His group is calling on the government to undertake urgent wider testing of plumbing products, and to regulate strongly.
The drinking water standard for lead was way too high anyway, said toxicology professor Ian Shaw - the maximum should be zero, given how bad low-level protracted exposure to lead could be to a person's neurosystem.
"In water that's a real issue because, of course, we drink water all the time, we use water to cook in, to wash in, and we're exposed to the lead in that water if it's present, over very, very prolonged periods of time - so that's the worst possible scenario."
Master Plumbers have failed before with their call for the previous Building Minister Nick Smith to take action.
The health minister referred RNZ's queries to current Building Minister Jenny Salesa, who declined an interview. In a statement, she said this was an operational issue for the Ministry of Business, Innovation and Employment, which would look at it as part of its current review of overall building product assurance systems.
"The current regulatory system for building products and assurance mechanisms for building products are voluntary - there is an over-reliance on voluntary mechanisms of compliance," Ms Salesa said.
"Conversely, there are limited instances where the greatest enforcement powers are used. The review ... aims to address the ... issues highlighted by this case."
MBIE would contact purchasers of the taps to highlight the safety concerns.
The alarm over lead in drinking water has been ringing louder, for longer, in Australia.
A 2016 New South Wales university study found low-level contamination in the drinking water of half the 212 homes tested, while 8 percent tested above the allowable 10 micrograms per litre.
"The taps contained [on average] 2.4 percent lead in the brass inside the tap and that certainly was adding lead to the water," said Elizabeth O'Brien, who leads an Australia-wide campaign for lead-free products.
The problems centred on brass taps under three years old - older taps tended to form a carbonate patina that stopped the lead leaching - and not just cheap ones, but also Watermarked products. Watermark is the main plumbing product quality scheme on both sides of the Tasman, but it too is voluntary.
The Watermark testing for lead were inadequate, Ms O'Brien said, and the joint trans-tasman standards on lead too high, but the Australian government was resisting any change.
"As long as people don't test their own water, or test their blood-lead levels, then that sort of government inaction will continue."
In New Zealand, there are hopes within industry that a review of National Water Standards might lead to a regulatory response to stop address leaching.
"If we can actually do something very soon to either monitor lead in tapware, monitor lead in water, and to stop the egress of lead into water, then I don't see a problem. If we don't do anything then there could be a problem because the exposure could be prolonged," Prof Shaw said.