A mass power shutdown across Melbourne's CBD triggered by a burst water main has caused peak-hour chaos in the heart of the city.Around 10,000 customers in the city's south-east were impacted and dozens of traffic lights in the area went dark after the water flooded a carpark on Little Collins Street, a Citipower spokesperson has confirmed.The water entered a substation in the basement, resulting in the power being cut in the CBD for safety reasons.NATIONAL: 'Surprise attack': Bondi probe hears