SYDNEY, Aug 11 (Reuters) - Australian authorities on Wednesday extended a lockdown in Melbourne for another seven days until Aug. 19 as authorities fight to get on top of the highly infectious Delta variant.

"If we were to open, then we would see cases akin to what is happening, tragically, in Sydney right now," Victoria state Premier Daniel Andrews told reporters in Melbourne.

On Wednesday Victoria state reported 20 new locally acquired cases of COVID-19 , a steady rate of increase. Victoria was plunged into its sixth lockdown since the pandemic began on Aug. 5, just weeks after exiting its last.

Neighbouring New South Wales is bracing for a rise in infections in Sydney a day after Australia's largest city reported a daily record of 343 cases. Of the cases reported on Tuesday, at least one-fifth were in the community while infectious.

The Delta variant outbreak, which started in Sydney, has spread to other regions of the state, sparking more local lockdowns, including in the popular tourist spot of Byron Bay.

Australia has so far fared much better than many other countries in the developed world during the pandemic, with just under 37,000 COVID-19 cases and 942 deaths.

However, the lockdowns in its two largest cities are derailing the country's plans for an economic rebound from the pandemic, with some economists forecasting its second recession in as many years.

Victoria state authorities lifted restrictions in regions outside Melbourne earlier this week after a slowdown in new cases.