Figures reported to the Cabinet Performance Management committee show that RCT Council's waste management services were £310,000 over budget during the first three quarters of the financial year.
The variation was said to be because the demand for bulky waste collections was 'reduced by more than anticipated' whilst there were 'temporary additional costs as a result of higher recycling levels.'
Welsh Liberal Democrat Councillor Mike Powell said this was not unexpected and showed a degree of naivety on behalf of the Cabinet.
"Did they seriously expect that once they started charging for bulky waste collection there would not be a drop off in the number of people using it? To base a budget on an assumption that people would just pay up and usage would stay the same is somewhat naïve.
"Likewise the whole aim of the changes to waste collection was surely to drive up the volume of recycling and reduce the amount of waste going to landfill and thus cut back in the associated costs of that. Didn't the Officer responsible foresee that?
"Of course you would expect that the upturn in costs for recycling would have been offset by a reduction in the costs of collecting black bag rubbish.
"Given the financial situation the Council finds itself in and the severe cutbacks it is proposing then the public have a right to expect that service changes are properly costed and budgets based on more than guesstimates."