SYDNEY, July 28 (Reuters) - Australian officials are set to extend a COVID-19 lockdown in the country"s largest city, Sydney, as new cases remain stubbornly high despite a month under strict stay-home orders, while Victoria and South Australia eased curbs from Wednesday. Sydney is struggling to control its worst outbreak of the year from a flare up of infections fuelled by the highly infectious Delta virus strain, with officials imploring residents to remain home except for urgent reasons. With the weeks-long lockdown due to end in three days, Australian media said authorities will announce a four-week extension on Wednesday after the tough curbs failed to bring cases down. New South Wales, of which Sydney is the capital, on Tuesday reported 172 new cases - its biggest daily rise since March 2020. read more The office of the premier of New South Wales did not immediately respond to a request seeking comment. Of particular concern is the growing number of people positive with the Delta strain moving around in the community. Unless that number returns to near zero, tough restrictions would continue, authorities have said. Around one in three new cases detected over the last several days have spent time in the community while infectious. As Sydney braces for tougher restrictions for most of August, Victoria and South Australia states came out of lockdown restrictions on Wednesday after getting on top of virus outbreaks. Australia has kept its COVID-19 numbers relatively low, with just over 33,200 cases and 920 deaths since the pandemic began, but the fast-moving Delta strain and low vaccination coverage have frustrated residents. Many Sydney businesses have been forced to shut, with the lockdown expected to take a heavy toll on Australia"s A$2 trillion ($1.50 trillion) economy. Finance Minister Simon Birmingham said the federal government could provide more support to employers depending on the decisions made by New South Wales authorities. "The (prime minister) will have more to say on that later today," Birmingham told the Australian Broadcasting Corp on Wednesday. ($1 = 1.3589 Australian dollars)