The signs are expected to be taken down by September, when the second stage of the Tahuna outfall and secondary treatment plant is complete.
But there may still be issues with pollution from other sources after that time.
For more than 100 years, Tomahawk and Smaills beaches were contaminated by sewage, and not usable, while St Clair, Middle and St Kilda beaches were regularly closed.
But since the Tahuna outfall pipe was commissioned on January 2009, tests of water from between Second Beach, near the St Clair pool, and Smaills Beach showed the water was safe for swimming.
The $37 million, 1.1km outfall pipe at Tahuna, commissioned in 2009, was stage 1 of a two-stage project, and the $75.8 million second stage was expected to be completed by September this year, later than the estimated finish time of May.
Stage 2 includes new ultraviolet disinfection facilities that will take the place of chlorine.
Council water and waste services manager John Mackie said this week the signs would stay while construction was under way, and chlorine was used to disinfect wastewater streams.
The ban on taking shellfish would be left in place in case anything went wrong during the construction phase, Mr Mackie said.
"It's a prudent measure."
Council monitoring in September last year showed no adverse environmental effects from either the Green Island sewage outfall at Waldronville or the Tahuna outfall, results on the council website showed.
All "sandy and rocky shore communities" examined were healthy.
But mussels and seawater were regularly sampled and analysed for microbiological contamination, and that showed there was some contamination of shellfish at Second Beach, Lawyers Head, Smaills Beach and Boulder Beach on "rare occasions".
Mr Mackie said once the signs were removed, there could be contamination from farming activities, and run-off into the ocean, especially during heavy rains.
Asked why the finish date for stage 2 was now September, wastewater treatment manager Chris Henderson said there had been delays.
Some delays were due to weather, others because of subcontractors affected by the amount of earthquake-related work required in Christchurch, and "just some things that have taken longer than anticipated".
