Regional council compliance manager Tami Sargeant said the council was still investigating the August 12 incident to understand what happened, what the effects to the environment were and whether enforcement action was appropriate.
The city council is building a new set of stairs for surfers to access St Clair Beach, over the rock armouring at the western end of the sea wall, near the St Clair Hot Salt Water Pool.
However, last month the contractor doing the work contacted the regional council to inform it the cement poured for the steps had not set and had washed away with the tide, Mrs Sargeant said.
Regional council staff went on site at the time and began an investigation to understand what happened and whether further action was needed, she said.
"The discharge of any substance into coastal or freshwater has the potential to create adverse environmental effects.
"Part of the investigation into this incident is to determine what effects may have arisen."
A city council spokesman said the contractor informed the city council immediately about the incident.
The spill happened because the wrong type of concrete was delivered to the site, the spokesman said.
"Our contractor acted to mitigate the spill once they became aware of this," he said.
The costs associated with the incident would be borne by the contractor.
However, the need for a combination of settled weather and spring low tides had led to some delays.
The council now expected to complete the access stairs by the end of October, the spokesman said.
