It is the second delay in the $100 million-dollar project, which was originally earmarked to finish this year.

Water NSW said the extension was required to carry out additional strengthening works on the spillway section, which fall outside the original scope to strengthen the wall in the case of extreme weather events.

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Water NSW said the project was 90 per cent complete and would be finished late next year. SRG is carrying out the works.

Ongoing works include installing post-tensioned cables on the concrete section of the dam wall; drilling inclined holes and installing post-tensioning cable anchors through existing concrete on the spillway piers; and stressing and capping anchors in the spillway.

Jobs already ticked off include the fabrication of the pier anchors; vertical drilling on the abutments and the spillway section; and the installation of post-tensioning anchors through the existing concrete into the foundation of the dam crest and spillway.

Water NSW said more than $3 million had been spent on local services including engineering, metal work, concrete supplies, light vehicle maintenance, waste management, crane hire, pumps and associated equipment, lifting and mechanical equipment, accommodation, and labour hire.

When the upgrades are complete, the dam will be compliant with NSW Dams Safety Committee standards for extreme weather events and seismic activity.

The Water NSW project is funded by the NSW government and public access to the wall cut-off until further notice.