By Robin Martin, RNZ
Fears parking sensors could explode have led New Plymouth District Council to replace or deactivate more than 500 of its 1600 sensors.
It has emerged the devices' lithium batteries are spontaneously combusting, causing them to blow up.
At least two sensors have exploded in the city recently and the council's scrambled to make sure it doesn't happen again.
The sensors are fixed to the ground and communicate with parking warden's hand-held devices to tell them when a vehicle has overstayed their welcome.
A problem with the sensors first emerged this month and electronics company A D Riley replaced 62 units.
But then another "malfunctioned" and a further 440 were deactivated.
In a statement, the district council said this "was a precautionary measure in the interests of public safety" but it did not elaborate.
When pressed, it revealed the batteries were overheating, providing an ignition source which caused "the sensor to break into fragments".
Chief executive Craig Stevenson this afternoon ditched the euphemisms and spelt out exactly what happened.
"A failing lithium battery vents and the pressure built up inside the sensor itself. It was greater than it was meant to be so it's lifted the top off the sensor.
"We've had two different circumstances so far, one where it's like a minor explosion I suppose you would characterise it, and the other was more of a loud bang."
He said a recently applied waterproofing gel had prevented failing batteries releasing pressure.
But it was not the first time a parking sensor has exploded in New Plymouth. In 2013, Rachael Walker and her daughter Susanne were shopping in Strandon when a sensor blew up under their car sending smoke billowing down Nobs Line.
Walker, whose car had to be towed away, told media at the time it was like "a bomb going off".
Stevenson said the public safety concern was genuine.
"The risk is that, you know, someone might have been standing too close when the top of the sensor came off. I think it's fair to say there was potential for injury and, of course, public safety is our paramount priority and that's why we've moved very fast and the company has been outstanding and also moved very fast."
He said all the failing batteries had now been removed and the remainder would be checked and replaced over time.
Councillor Murray Chong has long railed the introduction of the smart meters.
"We're wanting to get the best but it's not proven. I think we've spent $600,000 or $700,000 on replacing them a few years ago and then we had to replace them again just a couple of years ago and now these ones are playing up, so when do we learn our lesson. We should never have changed until we had proven technology."
He said lithium batteries were a known risk factor.
"For instance, if you took a mobile phone and you just punctured it, simply the air getting to a lithium battery will cause it to spontaneously combust, so that's scary if that's what's happening with our parking sensors and the batteries inside them."
On the streets of New Plymouth, folk were none-too-impressed to hear the parking sensors could blow up.
Michelle had reason to be concerned.
"I'm just parked over one of those sensors so I just hope it doesn't 'fragment' under my car."
Teresa Bloeman could imagine the potential for injury.
"Even if a car is parked over it or, you know, someone's walking next to it. It's dangerous either way."
Richard could imagine even worse.
"It must be a risk out there because if you have a leaky fuel tank or a bit of oil on the road there's going to be an explosion isn't there ... probably."
Unfortunately, Stevenson said the sensor issue did not mean shoppers could enjoy free parking in the run up to Christmas.
"The bad news there is they won't know which ones we're replacing, which ones we've turned off at the moment, so they should continue to feed the meters as they normally do.
"This is a very short-term issue, we will have them swapped out by next week at the latest and then the company is going to go through and upgrade all the batteries across the whole lot as a subsequent phase."
New Plymouth's 1600 sensors and 90 integrated pay machines were installed in 2012 at a cost of $1.2 million.
In 2017, it was discovered the sensors were not communicating properly with the wardens' devices, so last year 1500 of the units were replaced at a further cost to ratepayers of $800,000.
A D Riley is picking up the bill for the current repairs.