The two hours free parking offered is undercutting the council’s pay and display car parks, with councillors warned in January to expect losses of £197,000.
But costs have climbed to £275,000 by the end of the financial year, with the council losing £2,000 in potential parking income each week.
New parking charges, which were increased by 10 per cent, were not introduced until July 2018, meaning a full year of higher fees were not recouped.
The shortfall has also been blamed on the closure of two car parks in Abergavenny and Monmouth, both of which have since reopened.
During negotiations over the Morrison’s site, formerly the cattle market, the chain had been asked to apply the council’s charges for the 223-space car park.
But this requirement was removed in favour of a larger of a larger payment for the land, with the final deal being struck for £13.75 million upfront and £4 million spread over 25 years.
Speaking in January, a council spokesman said: “While negotiations were undertaken the Morrison chain was withdrawing from sites elsewhere as the supermarket sector contracted in response to structural changes in the market.
“Accordingly, our priority was the retention of the supermarket to increase footfall in the town centre and offset the likelihood of shoppers visiting neighbouring towns.
“It is inevitable that the creation of an additional 233 car parking spaces in Abergavenny would have an impact on car parking income regardless of whether charges are imposed, and we are delighted that the scheme as a whole has been such a success.”
The council’s operations department, which oversees car parks, overspent its collective budgets by £321,000 last year.
Maintenance of an ageing public transport fleet, combined with extra staffing costs, were the main source of losses alongside lost car parking income.
The figures will be presented to the council’s cabinet on June 5.