"The units have gone under numerous tests and the fault cannot be replicated," he said.
Nevertheless, the president of the Auckland Tramways Union, Gary Froggatt, was disappointed the employees were not advised there was a problem.
"We are pleased the batteries are going to be replaced and we are satisfied all health and safety requirements have been met."
But he said if the employees had been aware of the issue they may have been more likely to report other related issues, which may have sent up a red flag earlier.
But AT chief executive David Warburton said the agency had been in the process of addressing the problem before the incident on Thursday.
New fail-safe batteries had been ordered from French company SAFT, which specialises in making batteries in industries including aviation and defence. They had arrived this week and were about to be installed before Thursday's incident.
"We have now fast-tracked that and we will have all the batteries replaced with a new fail-safe circuited battery by the weekend," said Mr Warburton.
However, he was adamant the accident was a one-off, unpredictable incident.
"We didn't know it was urgent and we didn't know it was dangerous."
The explosion, which damaged the bus' rear-view mirror and cracked its windscreen, was related to a malfunction with its ticketing console that caused the internal AA-lithium battery to overheat, explode and cause the plastic casing to fly off.
The driver went to the emergency department to be treated, but was expected to return to work next week.
There were passengers on board the bus at the time, but they were not hurt.
Technicians are being flown in to fit the new batteries to all buses.
Mr Warburton promised no services or customers would be affected while the replacement took place.
He said each battery would be replaced on the buses' return to the depot - a process that would take only 15 minutes.
AT said a full review into the incident would be carried out.
