SYDNEY, Aug 17 (Reuters) - COVID-19 cases are set to "rise substantially" in Sydney in the coming weeks despite a prolonged lockdown, authorities said on Tuesday, warning soaring infections have already put hospitals under enormous strain. Australia, once a world leader in curtailing COVID-19, is struggling to suppress a third wave of infections driven by the highly infectious Delta variant despite locking down more than half its population. New South Wales (NSW) state, whose capital Sydney is the epicentre of the latest outbreak, reported 452 cases in the past 24 hours, the third-biggest one-day jump, and one new death. "We envisage that case numbers in the next two or three weeks will bounce around and are likely to rise substantially," NSW Premier Gladys Berejiklian told reporters. Sydney has already toughened restrictions, including setting up roadblocks in parts of the city, and has hiked fines amid reports of people flouting strict stay-at-home orders. Economists fear the lockdowns may drive the country"s A$2 trillion ($1.5 trillion) economy into a second recession in as many years, although Australia"s central bank stands ready to take policy action, minutes from its August meeting showed on Tuesday. read more In Sydney"s south, 80 medical staff were forced into isolation at a local hospital after several cases of COVID-19 were found in an oncology ward, NSW Health Minister Brad Hazzard said, noting reports of ambulances queuing to access care in the city"s west. "There"s no question that the hospital system is under enormous pressure across New South Wales," Hazzard said. COVID-19 SPREADS As the virus spreads to regional centres across NSW, a case was detected as far away as Broken Hill, a mining town more than 900 km (560 miles) northwest of Sydney deep in Australia"s outback, raising fears of wider outbreaks. The country is racing to speed up its sluggish vaccination rollout, with only about 26% of Australians fully vaccinated. In Melbourne, the capital of Victoria state, 24 new local cases were detected on Tuesday, the first day after officials reinstated a night curfew and extended a hard lockdown until Sept. 2. Twenty-two cases were reported on Monday. Australia"s capital, Canberra reported 17 new locally acquired COVID-19 cases and authorities warned infections would likely rise in the coming days. Despite the recent Delta outbreaks, Australia"s coronavirus numbers are well below than many other countries in the developed world, with just over 40,000 cases and 967 deaths, but its vaccination figures are among the lowest. A total of 57 people, most of them unvaccinated, have died in the latest outbreak in Sydney since July 11. There were no COVID-related deaths in Australia during 2021 until then. Officials have been procuring emergency vaccine supplies from abroad and expect the pace of inoculations to pick up once more supplies arrive from October. read more ($1 = 1.3678 Australian dollars) Reporting by Renju Jose in Sydney and Colin Packham in Canberra; Editing by Sonya Hepinstall and Richard Pullin Our Standards: The Thomson Reuters Trust Principles.