The fate of the Queensland town of St George rests on a makeshift flood levee.
Thousands of residents were forced to evacuate the town after it was predicted the Balonne River would reach 15 metres.
The ABC reports the river was at 13.7 metres on Monday night and is expected to go close to the levee height of 14.5 metres.
Police estimate 30 homes have been inundated so far.
In New South Wales, about 7000 people are now cut off by swollen rivers.
The ABC reports several communities, including Walgett, Bourke, Lightning Ridge and Brewarrina, could face days or even weeks of isolation.
NSW State Emergency Services says supplies will be ferried into these areas until the water recedes.