"This will be further compounded by the additional housing proposed in an application for a SHA."
The treatment plant's consent expires in 2022 and the district council is planning to pipe Lake Hawea sewage to its main Upper Clutha treatment plant, Project Pure, at Wanaka Airport.
A council spokeswoman said construction was "currently scheduled to begin in 2020".
"The Project Pure plant needs to expand before Hawea can be connected."
She did not respond to a request for a timeline.
Ms Mackenzie said it was her understanding the pipeline was unlikely to be completed for at least four years.
The council is considering three options to deal with wastewater from the SHA: a temporary on-site treatment plant, transferring it to Project Pure by truck, or storing it during the day and pumping it to the Hawea plant during periods of low flow.
A decision on whether the housing area should be allowed to go ahead is due to be made by Associate Minister for Housing and Urban Development Jenny Salesa.