A state of emergency was declared for the remote Bluecliffs community on Thursday due to concerns of rapid erosion from the sea and Waiau River, with metres of land already lost.
Eighteen properties are affected.
Group controller Simon Mapp said he expected the community would start to notice the reduced flow on Tuesday afternoon.
The lower river flow would also help workers who were attempting to create a new opening to the Waiau River to direct the flow away from the properties this week, he said.
"It's not like a normal sand bar, I suppose. It is metres high so there's a lot of material that's got to be shifted and weather conditions, health and safety concerns are paramount and therefore, they determine the amount of work that can be done on any given day."
They hoped to have the bar open as soon as possible - but by Saturday at the latest, he said.
MetService has issued an orange heavy rain warning for Fiordland north of Doubtful Sound from Wednesday night until Thursday morning, with up to 100 millimetres of rain forecast.
It was expected to have some impact on the lakes that feed into the Waiau River, but not too much on the community, he said.
"That does put a bit of pressure on and we'll just keep a wary eye on the river flows coming down."
Residents had not reported any more large chunks of land being eroded away in the last few days, Mapp said.
"They are a hardy bunch, but you can understand they've been under stress for quite some time so we've talked to them all and afterwards I felt there was a relatively good mood and it was relatively buoyant.
"But you can tell that they've been under a lot of stress and they will remain under stress for quite some time."
Last week, the properties were all white-stickered meaning they were safe to live in but residents should be prepared to evacuate at short notice.
The Southland District Council was bringing in experts to assess and come up with a plan for an old community tip next to the river following earlier allegations there could be explosives in it.
Contractors cleared the seaward end of the tip before the allegations surfaced last September.