Millions of locusts swarmed towards Australia's second biggest city of Melbourne on Thursday, as the insects were also reported near the southern city of Adelaide.
Brought to life in February by drought-breaking rains, billions of locusts first swarmed along a 1,200-km (745-mile) front from south-west Queensland state to the central New South Wales town of Dubbo, across an area twice the size of England.
The move to major cities by the crop-devastating insects widened the battlefront in Australia's three-month effort to contain the swarms.
Despite an aerial spraying campaign since February, the locusts have now spread a further 700 km (435 miles) south-west to establish a five-million-strong swarm north of Melbourne.
"We had somebody phone up this morning to say they had a locust in the back garden," said Laury McCulloch, a director of the Australian Plague Locust Commission.
Officials in Adelaide also confirmed the arrival of locusts.
Numbers in the outback have not been officially estimated, but are recognised as the most serious since up to 100 billion locusts swarmed in late 2000.
The return of very dry weather to much of New South Wales has driven the locusts to search for more favourable conditions in the south, said McCulloch.
Aerial spraying is now in progress in hot spots north of Dubbo, a town on the edge of Australia's eastern population zone and which was invaded by the insects last month.
The swarm 80 kms north of Melbourne probably did not justify aerial spraying, but was being monitored, officials said.