Electronic consoles on more than 1000 Auckland buses are being urgently modified after one exploded and injured a driver.
Auckland Transport said the driver has hurt by flying glass, after the AT HOP electronic ticket machine blew up yesterday while he was driving.
The force of the blast was so great that the windscreen was cracked and the driver had to be taken to hospital.
Auckland Transport said yesterday's malfunction and explosion was one that has never happened before.
But it said it did know of one other unit overheating late last year, however at the time it was treated as a technical, rather than an urgent problem.
The fault has been traced to the ticket machine's internal lithium battery, and these are now being modified.
The explosion not only cracked the windscreen, but also broke the bus rear-vision mirror.
The union for Auckland bus drivers said it should have been told months ago that the ticket machines they used could overheat.
Gary Froggatt, from the Auckland Tramways Union, said the first fault in December should have been treated as a health and safety issue.
"He got a hell of a fright. It could have been much worse, and perhaps if there been a little kid there or another passenger at the machine then the passenger could have been injured as well."
Mr Froggatt said the driver could have been hurt much more seriously.
Auckland Transport chief executive David Warburton said the batteries were being urgently replaced by Thales - the company which made the consoles.
"Thales have not had this experience before on any unit, it's extremely rare, Thales are working with us to ensure that the problem never occurs again and we are relooking at the reinstallation of the units as well as the change on the battery supply."
Auckland's mayor meanwhile has demanded answers.
Mayor Len Brown said he understood Auckland Transport knew changes might be needed to the machines, but he wanted to know what information it had from the manufacturer.
It is the first setback involving the city's new $100 million electronic ticket system, which now operates on all public transport services.
Auckland Transport said technicians from the system supplier Thales had arrived from Sydney, and would fit upgraded batteries to machines in up to 1200 buses over the next day.