Ground penetrating radar has revealed numerous cavities beneath the University Reservoir at Smithfield, which will need to be filled before the asset can be brought back online.
The 20-megalitre reservoir was drained late last year due to a significant water leak that was undermining its footings and structural integrity.
Council will inject grout into cavities to stabilise the slab beneath the reservoir, before coating the internal walls of the structure to address leakages.
The reservoir will then be slowly filled and monitored to determine if the leaks have been repaired and assess the integrity of the structure.
Council expects the reservoir will remain offline until mid-March.
Residents in the northern beaches, from Yorkeys Knob to Clifton Beach, may experience a reduction in water pressure until the repairs are completed and the reservoir is back online.