As much as 100mm of rain has been predicted across already soaked catchments, with the Bureau of Meteorology expecting up to 50mm of rain for Bendigo.

Coliban Water operations manager Neville Pearce said the authority would have staff on standby as it prepared for inflows into already overflowing storages.

“Our storages are effectively full – we’ve only got one that’s got six centimetres in terms of space,” he said. “We’ve got 25,000 megalitres more water than we’ve ever had before.”

He said any inflows into storages above Malmsbury would make their way down the Coliban River and combine with the Campaspe system in Lake Eppalock.

Mr Pearce was most concerned about inflows into the sewerage system, which has struggled to cope already this year.

System failure could see sewerage overflow into backyards and manholes or prevent toilets from flushing.

“Most of our emergency response will be to manage any inflows... management of sewer systems is our biggest concern.”

Goulburn-Murray Water regulated systems acting manager Andrew Shields said inflows would depend on how quickly rain fell and the areas affected.

“We’ll have to see how it transpires and keep an eye on it as it occurs,” he said.

“Tullaroop and Laanecoorie are both at capacity so they’ll fill and spill,” Mr Shields said.

Lake Eppalock is at 70 per cent capacity and is expected to rise even further as the weekend rain makes its way downstream, while on the Loddon River, Cairn Curran is at 90 per cent.

Mr Shields said storages in the Loddon and Campaspe regions were the highest in almost a decade and already well placed to deliver water for irrigators and other customers heading into summer.

Bureau of Meteorology senior forecaster Scott Williams said rain would fall in the Bendigo region from midnight tonight.

“There will be a reasonably substantial amount of rain mid to late morning Saturday,” he said.

Mr Williams said 15 to 20mm was expected to fall during the day tomorrow with another 20 to 30mm overnight and in the early hours of Sunday.