Australia's second largest city Melbourne reinstated Covid-19 restrictions as authorities scrambled to find the missing link in a fresh outbreak that has grown to five cases, Reuters reports.

Home gatherings will be limited to five guests, only 30 people allowed at public meetings, and face masks will be mandatory indoors from 6pm local time on Tuesday until June 4.

“This is a responsible step that we need to take to get on top of this outbreak,” James Merlino, Victoria state's acting premier, told reporters in Melbourne.

The latest Melbourne outbreak ends Victoria state's run of zero cases for nearly three months.