MetService issued a rare red heavy rain warning for the city on Sunday.
After dark the downpour intensified, leaving streets under water, closing roads across the Taieri and rapidly pushing up the Silver Stream water levels.
As the water rose on Sunday night in Mosgiel, a whole town rose with it.
Mosgiel-Taieri Community Board chairwoman Rebecca Shepherd said it was an "awful night".
"I feel very sorry for the people that got water in their houses," Mrs Shepherd said.
"It is frightening for people."
As the rain worsened, she investigated the situation, taking a drive at about 5.30pm and finding several roads already awash as the drainage network began to struggle.
By about 6pm, the drain carrying water from Reid Ave into Silver Stream was at capacity and surface flooding had spread around Green St.
She understood up to 10 families were evacuated or chose to leave, mainly from the Tay St and Tyne St area not far from Silver Stream.
"They had surface flooding all through there and one of the houses of their neighbours, the house actually had water through it inside."
Silver Stream began to spill, but the rain eased before it could break its banks.
"Apparently it did start to spill at the spillway but the rain obviously slowed down enough that it did not do any of the devastating things it could have done.
"It looked to me like it was doing what it was meant to do," Mrs Shepherd said.
Locals were quick to mobilise.
The Mosgiel Taieri Community Resilience Group alongside the Mosgiel-Taieri Community Board opened an emergency hub at the Taieri Bowling Club in Wickliffe St just before midnight.
The Red Cross helped set up beds at the base and the local Pizza Hut donated food.
One family whose street was surrounded by water stayed the night, returning home once Mosgiel-Taieri Community Patrol volunteers had confirmed their house was safe.
The Lions Club of Mosgiel organised a sandbagging effort and waves of helpers kept filling bags through the night, even after the sand briefly ran out and more had to be delivered.
While filling bags, Mrs Shepherd spoke to a woman who always headed down to help whenever she knew something was happening.
"No-one asks her, she just goes and does it.
"Those sorts of things are amazing."
The community patrol’s volunteers worked through the long list of closed roads to see whether they could safely be reopened and inspected individual houses to confirm residents could return.
She was impressed by the many civic-minded people in the area who stepped up to assist.
"When something like this happens it really shows those community groups that aren’t necessarily in the limelight and you don’t know about them until you are involved and need them," Mrs Shepherd said.
