The $1.2 million lids for the Clive East wastewater plant may not entirely contain smells, the Hastings District Council has been warned.
A report to the council's finance and operations committee said there may be underlying odour problems that have been masked by the hydrogen sulphide smell from the tanks.
The report said it would be impossible to tell whether there would be future odour problems, and more work would be needed to assess the impact of the covers.
Council chief executive Ross McLeod said he was still confident the lids would be an effective solution.
"The advice we've had from the experts is that the lids and chemical treatment should fix the problems, but they have, as all experts do, given themselves a little disclaimer that there may be other issues underlying," he said.
"It may not be the absolute solution but the advice we have is that it should work and we're still working on that basis."
He said the chemical dosing currently being used to reduce smells, which would continue once the lids were in place, had already been effective in reducing complaints.
There were still a small number of complaints during hot weather and particular wind conditions.
The biological trickling filter plant, which uses bacteria to break down wastewater material, was built in 2009.
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It prompted ongoing complaints from nearby residents because of smells.
The council has commissioned lids to be built and added to the tanks to contain the odour. They are expected to arrive before Easter.
Concerns were raised at the meeting that import duties had not been included in the $1.2 million budget, but the council said that cost had been accounted for.