Details about cash paid to the mobile company emerged at an audit meeting held jointly by the force and crime commissioner Winston Roddick.
Mr Roddick said steps were being taken to recover the money,
Clwyd West AM Darren Millar said he was "astounded" by the oversight.
"Taxpayers will want to know how North Wales police missed that they were spending  thousands of pounds  on a service that wasn't used," the Tory AM said.
"If this means that local taxpayers have paid more on their police precept than is necessary then it is unacceptable.
"Taking two years to discover this brings into question the checks and balances in their internal audits, and at a time when every penny counts."
The force was told the unnamed mobile phone operator accepted there was "a likely issue" and repayments were being agreed. The error was discovered at the end of August, with auditors told 12 days later.
Only last week the force was warning about effects on policing and security if Chancellor George Osborne cut its budget.
Mr Roddick said in a statement: "I am pleased to say that this discrepancy was picked up by the force's own financial department and steps are now being taken to recover the money involved.
"I was made aware of the situation in September and have been monitoring the force's negotiations with their mobile phone service provider.
"As it was only identified in the current financial year it hadn't appeared in annual accounts but it does underline the need for vigilance in financial affairs, particularly when they involve the public purse."
A police spokeswoman said: "We are in talks with the company concerned and the matter is being resolved."
North Wales Police spent £560,000 over two years for a mobile phone service it never used before the mistake was spotted.