Epsom and St Helier hospitals accidentally overpaid staff nearly £300,000 last year.
A freedom of information request revealed that on top of £223,941,000 in salary payments last year the Epsom and St Helier trust mistakenly overpaid staff £294,000.
The trust, which currently employs 4,502 people, said any miscalculation of pay was taken very seriously.
During the financial year 2011-12 they accidentally overpaid staff by £178,000 but the trust said they have already recovered £142,000 of that money and were actively pursuing the remaining debt.
Tom Brake, the MP for Carshalton and Wallington, said it was essential the trust keeps a tight leash on overpayments and added: "With the demand for health care as strong as it is, every penny that's available needs to get spent on patient care."
Jonathan Isaby, political director of the TaxPayers' Alliance, said: "It’s deeply worrying that the NHS trust running Epsom and St Helier hospitals is throwing hundreds of thousands of pounds a year at staff in overpayments.
"Taxpayers will be concerned that the hospital trust appears to have been so blasé when it comes to administering staff salaries, especially at a time when budgets are so tight, and will wonder how else their cash is being wasted."
He warned hospital bosses to get their act together to get the money back and ensure systems were in place to prevent overpayments happening again.
A spokesperson for the Epsom and St Helier trust said: "Salary overpayments can unfortunately occur due to a number of reasons in a workforce of thousands, due to contractual changes, maternity leave, national insurance and tax alterations.
"However, as can be seen from our financial year 2011-12, we actively pursue overpayments via debt collection agencies who have demonstrated a very successful recovery rate."