WELLINGTON (Reuters) - A power blackout brought chaos to New Zealand's Auckland city on Monday, leaving thousands of houses and businesses without electricity, roads gridlocked, phone lines down and hospitals closed. The New Zealand Press Association said strong winds from a cold front that began sweeping northwards on Sunday had snapped a power line at the Otahuhu substation, the main supplier of power to the country's largest city from the south. Transpower spokesman Chris Roberts told NZPA that an earth-wire at the substation had fallen across a 110kv feed, one of two main feeds into Auckland. About half of the region -- including most of the south and central Auckland -- were without power. The blackout halted trains on the suburban commuter rail network, traffic lights failed and some central city businesses were evacuated, NZPA said. Auckland Mayor Dick Hubbard said the city council had initiated civil defense procedures. "We're not in a state of civil defense but we are on alert," Hubbard said. "We've initiated first level civil defense just in case there is a major problem." "This outage affects two-thirds of the area of the city, which would be 700,000 people," Hubbard said. Two of the largest hospitals in Auckland were closed due to the blackout, the country's Health Ministry said in a statement. Auckland International Airport, the country's main gateway to the rest of the world, was also affected by the blackout, though emergency generators provided enough power and there was no disruption to flights, the airport company said.