SYDNEY, Aug 19 (Reuters) - Australia"s Victoria state on Thursday reported 57 locally acquired cases of COVID-19, the biggest daily rise this year, as authorities scramble to quell an outbreak of the highly infectious Delta variant in Melbourne, the state capital. Of the new cases, 13 spent time in the community while infectious, the state"s health department said. The source of three cases is unknown. Victoria reported 24 local cases on Wednesday. Reporting by Renju Jose; editing by Diane Craft Our Standards: The Thomson Reuters Trust Principles.