MELBOURNE—Auckland, New Zealand’s biggest city, woke up on Sunday morning to a second lockdown in a month as health authorities try to rein in a coronavirus cluster of the more contagious UK variant. The seven-day lockdown, announced late Saturday by Prime Minister Jacinda Ardern, follows a three-day stay-at-home order in mid-February after a local emergence of the UK variant of the new coronavirus that causes COVID-19. Genomic sequencing of a new case recorded on Saturday, which prompted the lockdown, was linked to the existing cluster, health authorities said on Sunday, bringing it to 13 cases. “It’s unlikely we wouldn’t see more cases,” COVID-19 Response Minister Chris Hipkins told the state-owned television network TVNZ on Sunday. “How many cases at this point we simply don’t know.” The new case has been considered infectious for a week. The person, a 21-year-old student, visited a number of public spaces during that time. The …