The bureau's manager of long-range forecasting, Andrew Watkins, said daytime temperature throughout winter would be cooler than average across the southern part of the country. But overnight temperatures were likely to be warmer than average across the entire country.

"Most areas of mainland Australia are showing a better than 70 per cent chance of having a wetter than average winter," Dr Watkins said.

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He said the outlook was being largely driven by warmer than average ocean temperatures off north-west WA, as well as warm ocean temperatures in the western Pacific.

"Areas with a higher chance of above average rainfall are also looking at increased chances of cooler than average days because they are more likely to have cloud cover and more evaporative cooling in the coming months," he said.

"Conversely, it means our nights are more likely to be warmer than average, because that cloud cover will prevent heat from escaping during the evenings."