NZTA denied the bungle cost taxpayers but admitted it did not keep track of such expenses.
It is now in talks with Google about a possible data breach.
Traffic Watcher was accessed 600 times in March and July this year but almost 3000 times in May.
NZTA has not confirmed if the May surge was due to the insecure key, did not say when it finally secured the key, nor has it provided the earlier site usage figures.
However, it has confirmed to RNZ that it corresponded with Google about a breach or possible breach of data storage.
Google declined to comment.
RNZ's OIA request for details was immediately rejected by NZTA on commercially sensitivity grounds.
"There was one known attempt by a contractor to use this API, which Google shut down as part of their management and security processes, and so stopped access," NZTA said in a statement.
However, Traffic Watcher was developed by the Connected Journey Solutions unit, which a recent independent review found "was given an extraordinary degree of freedom" that was abused in multiple ways.
Ninety per cent of the unit's 100 or so staff were short-term contractors who were allowed to use personal computers and personal emails to do work, and misused NZTA domain names.
"Shadow technology, inconsistent identity and access management processes, and a lack of technical and architectural input have led to vulnerabilities in security and resilience," the review said.
There is industry speculation that misuse of the API key was widespread, that contractors took the key's details with them when they left, and that the bill being sheeted back to NZTA was high.
The agency denies this: "At no time has NZTA faced increased costs over its licenced amounts for access through Traffic Watcher, nor has the agency incurred any additional costs as a result."
But a separate OIA response from NZTA shows:
• It did not keep track of Traffic Watcher data costs.
• It did not keep track of the cost of research and development on Traffic Watcher from February to December 2018.
• It could not disclose these costs because they were not individually accounted for, it said.
• Traffic Watcher's development costs this year to June were $375,000.
The independent review in July does not mention the API problem.
But it said financial governance at Connected Journeys was lax so funds were not clearly accounted for. There was an inability to accurately identify expenditure.
"A lack of oversight undermined [the unit's] ability to deliver and operate quality products," it said.
NZTA's contracts with Google contributed to a 75 per cent leap in its software licensing fees last year, up from $4m to $7m. Its data access and storage fees are not recorded.
NZTA would not provide details. It cited commercially sensitivity for refusing RNZ's OIA request to disclose:
• What its three Google contracts are worth.
• The operating costs for the Google data or cloud contracts.
• What it has paid for data services in total since 2013.
• The Google contracts do not appear among the hundreds of NZTA contracts that have been made public.
Google got the contracts directly and there was no public tender. NZTA said it followed its procurement rules.
Transport Minister Phil Twyford's office said he was made aware of the Traffic Watcher app, and the costs and problems at Connected Journeys, as part of the July review.
Connected Journeys circumvented many public sector controls, with the knowledge of former NZTA chief executive Fergus Gammie, the review found.
It was shut down earlier this year after Gammie and unit director Martin McMullan resigned.
