More questions are being asked about the reliability of Dunedin's street lighting after a delay in turning on the lights resulted in a citywide street light blackout for at least half an hour last night.

Delta marketing and communications manager Gary Johnson said the lighting system had come on about half an hour later than planned yesterday, with the lights switched on manually about 9pm after automatic switching had failed.

The Otago Daily Times received several telephone calls from residents pointing out that street lights were not working.

After the last blackout on October20 - the city's third street light blackout in less than two months - the Dunedin City Council said it was seeking answers from Aurora Energy Ltd.

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Contractors had to manually switch on blocks of lights one by one.

This followed a citywide blackout on September 16 and a more limited outage on September24.

Mr Johnson said the previous faults had been fixed and the latest problem would be fully investigated.

Dunedin city councillor Lee Vandervis said last night he was concerned about the repeated outages, because deeper problems, such as ageing infrastructure, could be behind them.