Anywhere from 250 to 1000 homes could be inundated in Rochester, depending on the severity of flooding.
Rochester SES controller Tim Williams said the expected situation would leave 250 houses with water above floor level.
He said he didn’t know exactly how many houses would be impacted by a worst-case flood.
“We haven’t had one that big,” he said.
“In 2011 there were 700 houses which had water through them, and this time it’s predicted there could be 100mm more.”
He said the extra 100mm from 2011 could lead to an extra 300 houses being affected.
“That 100mm potentially could mean 1000 (houses impacted) in the worst-case scenario,” he said.
Mr Williams said 11,000 sandbags were filled in Rochester on Wednesday and another 10,000 were filled at Rochester Recreation Reserve on Thursday.
Another 7000 sandbags were filled at the Rochester swimming pool site as of 3pm on Thursday, with more being filled and another 5000 sandbags being trucked into town on Thursday night.
“When it’s done tonight, it’s done,” he said on Thursday.