MELBOURNE—Sydney began a two-week lockdown on Sunday as a cluster of cases of the highly infectious coronavirus Delta variant rose to 110 in Australia’s largest city, while an outbreak in the northern city of Darwin prompted a hard two-day stay-at-home order. “Given how contagious this strain of the virus is, we do anticipate that in the next few days, case numbers are likely to increase beyond what we have seen today,” News South Wales state Premier Gladys Berejiklian told a news briefing in Sydney. The island nation has been more successful in managing the pandemic than many other advanced economies through swift border closures, limited international travel, and social distancing rules, reporting just over 30,450 cases and 910 COVID-19 deaths. But the country has confronted an increasing number of small outbreaks in recent months. These have been contained so far through speedy contact tracing, isolation of thousands of people at …