Civil Defence controller Simon Mapp said work to cut into the gravel bar and open the mouth of the Waiau River was completed last night.
The work, which involved moving 30,000cu m of material, aims to realign the river opening to the sea in the hopes it will delay further erosion in front of the Bluecliffs coastal community.
It was "unknown how successful this opening will be or for how long it may last", Mr Mapp said.
Eighteen properties are affected, although only six are permanent residences.
There was also an old community tip that may also contain possible explosives that was exposed to the erosion.
"We are currently awaiting the weekend’s increased river flow and incoming weather to determine the effectiveness of the bar opening.
"This work has always been a struggle against Mother Nature, and the team has done a commendable job despite difficult circumstances," he said.
Meridian Energy had been managing the river flow release at the Manapouri Lake Control structure but over the next two days there would be a gradual increase in the release of water from the control gates.
Forecast rain would also contribute to the flow in the Waiau River.
"The area around the new opening in the bar east of Bluecliffs is extremely dangerous, so please do not go on to or near this area.
"Please continue to avoid any unnecessary travel to the area while this state of emergency is in place," Mr Mapp said.
The bar opening would hopefully reduce the erosion in front of the properties and the tip, which will enable further discussions with the community about the future of the area.
The state of emergency has been in place since February 8.
