Christchurch residents may have to wait until September to have a chlorine-free water supply.

Christchurch City Council is still repairing and resealing 95 underground well-heads found to be vulnerable to contamination.

Water supply improvement manager, Helen Beaumont, said while the repairs are picking up as water usage drops, dropping the chlorine will still take some time.

"It's not exactly on schedule. We'll be working progressively all the through June to September, and the chlorine will come off as soon as the work has been done."

She said she's reasonably confident in getting the last of the chlorine out of most of the city supply.

However, Beaumont said the central southern suburbs, around Spreydon and St Martins, are a challenge.

"I haven't got an exact timetable for the work through there, as we're still doing some modelling about the best program of works through there, and how we can get water from other parts of the city across into those zones and how we can reconfigure the mains."