The former prime minister said undeclared spending in the 2015 general election amounted to only around 0.6% of the Tories’ total budget for the campaign, and that the party had kept well within the permitted spending limit.
Former Tory treasurer Simon Day is facing a police investigation after an Electoral Commission report found “numerous failures” in Conservative reporting of expenses for the general election and three by-elections in 2014.
The commission’s findings also “cast doubt” on the expenses declarations of individual Conservative candidates.
They include Craig Mackinlay, who successfully saw off the challenge of former Ukip leader Nigel Farage in South Thanet, as well as MPs visited by the party’s controversial Battlebus2015 tour.
Asked about the findings at a charity event in London, Mr Cameron said: “In terms of a party being fined, this is not the first time this has happened. Other parties have been fined.
“In terms of the percentage of spending that wasn’t declared that should have been declared, it was something like 0.6% of total spending. We spent well under the cap for the election spending limit.”
The former prime minister, who quit in 2016 after losing the EU referendum, added: “I’m very glad that we do have robust election spending limits and robust ways of policing these things in this country.”
Mr Day, who was party treasurer until April last year and signed off the party’s expenses declaration, has been referred by the commission to the Metropolitan Police.
He could face charges under the Political Parties, Elections and Referendums Act if he is judged to have “knowingly or recklessly” made a false declaration that the party’s election spending return was complete and correct.
The commission’s findings will alarm Conservative MPs who have already faced police questioning over their individual declarations.
At least 12 forces are known to have submitted files to the Crown Prosecution Service with a view to possible criminal action.
It comes with the Tories under pressure on numerous fronts, including a dramatic U-turn on Budget tax rises, the Scottish Government’s demands for a second independence referendum, and as Theresa May prepares to trigger Brexit by the end of the month.
The Prime Minister said the party would pay the fine imposed by the commission. “We have complied fully with the Electoral Commission throughout their investigation,” she told ITV News.
But the tensions within the Tory ranks were underlined when party chairman Sir Patrick McLoughlin reportedly tried to knock the mobile phone from the hand of a Sky News journalist as he tried to film him outside the Houses of Parliament.
Altogether, the commission found payments worth at least £104,765 were missing from the party’s general election expenses declaration while payments worth up to £118,124 were either incorrectly reported or not reported at all.
They included spending of £63,486.83 which the party failed to report in relation to the Battlebus2015 tour which bussed activists into around 30 key target seats in the final weeks of the campaign.
The commission said a “proportion” of the spending on the tour should have appeared in the individual returns of the candidates visited, rather than the party’s national expenses, “casting doubt on the accuracy of those returns”.
It also found that some of the spending clocked up by a team of four senior national party officials based in South Thanet during the campaign should have been included in Mr Mackinlay’s expenses, raising “doubt as to the accuracy and completeness of his expenses return”.
Although the officials were not named by the commission, they have previously been reported to include Mrs May’s trusted chief of staff, Nick Timothy.
In imposing the fine, the commission - which at one point had to obtain a court order to get the information it needed - said it had taken into account the “lack of co-operation” by the party with its inquiry.
Commission chairman Sir John Holmes said the Tories’ failure to follow the rules “undermined voters’ confidence in our democratic processes” and suggested they may need tougher powers to police the system.
“There is a risk that some political parties might come to view the payment of these fines as a cost of doing business,” he said.
“The commission therefore needs to be able to impose sanctions that are proportionate to the levels of spending now routinely handled by parties and campaigners.”
A Conservative Party spokesman said both Labour and the Liberal Democrats also had been fined in relation to their general election expenses, while the Lib Dems were facing a police investigation.
“This is the first time the Conservative Party has been fined for a reporting error. We regret that and will continue to keep our internal processes under review to ensure this does not happen again,” the spokesman said.