This “has the potential to lead to widespread flooding” across an already soggy east coast.
“Above average rainfall also increases the risk of landslides and tree falls in areas with steep terrain and very wet soil,” the BoM said.
There is a high chance of warm nights in the tropical north and in some southern areas, including Tasmania.
Meanwhile, cooler days are expected for New South Wales, parts of southern Queensland and into Victoria.
There is always a risk of bushfires in South Australia in the summer, and above-average rainfall could increase the risk of bushfires, the BoM said.
The Summer Bushfire Outlook is published on 29 November.