Sydney's poorest neighborhoods on Friday braced for military enforcement of the city's toughest and longest lockdown of the COVID-19 pandemic as the infection numbers held persistently high five weeks since restrictions began.

With the city of 5 million people ordered to stay home amid an outbreak of the highly transmissible delta variant, authorities outlined even tighter restrictions for the worst affected suburbs, including mandatory testing and mask-wearing outdoors.

From Monday, some 300 Australian Army personnel will help police door-knock people who have tested positive with the virus to ensure they are isolating, New South Wales police commissioner Mick Fuller said at a televised news conference.