In 2018, the council bought a portion of 15 Maiden Smith Dr to construct a stormwater retention basin and pump station.
Works began in August last year and are due to be completed this month.
MRC assistant manager of finance Kris Kershaw said the original budget was set as $880,000.
However, a review found the estimated cost to be $1,275,014, resulting in a $413,154 deficit.
More than $250,000 of this was due to “inadequate budget for the land purchase and scheduled works to be done”, the assistant manager of finance said.
The rest was a result of “unexpected site conditions” which saw a variation to the scope of the works.
In last week's council meeting, Cr Nikki Cohen said the increase was “a bit of a worry”.
Cr Gen Campbell agreed and said council needed to pay more attention to the risk analysis.
“I have to question who was the project manager on this that we’re that far out of the budget,” she said.
“I’m very concerned because this is quite a significant shortfall.”
MRC chief executive Des Bilske said the project highlighted the need to change procedures associated with monitoring expenditure during projects.
The extra funds will be made up from an allocation from the industrial land internal reserve fund and internal borrowing from the sewer fund.
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