Planned testing of tsunami sirens along Christchurch's coast did not happen because the Auckland-based person responsible was dealing with a car crash outside their property at the time.
Christchurch City Council initially blamed the failure on "human error" but on Tuesday provided further details about why the 45 sirens between Taylors Mistake and Brooklands did not sound at 11am last Sunday morning.
Head of community support and partnerships John Filsell conceded it was unacceptable to have an emergency system so dependent on one person.
"The supplier was providing immediate response to a motor vehicle accident outside their property at the time of the test," he said.
"We recognise that it is not acceptable to have a system that relies so heavily on one individual, which is why discussions are underway with our external supplier to ensure there are sufficient safeguards in the system to reduce the risk of this happening in the future.
"We are also currently reviewing the role of sirens in the tsunami warning system for Christchurch and Banks Peninsula. The issues experienced on Sunday will form part of that review."
Testing of Christchurch's tsunami sirens usually coincided with the beginning and end of daylight saving time.
The sirens were expected to be retested in the next couple of weeks.
In a memo sent to community leaders on Monday, the council's civil defence and emergency management manager Brenden Winder said civil defence could not directly access the siren system.
The council first hired the external supplier in 2012.
RNZ had contacted the council for further information about the failure.
