Soaring police overtime in Nassau, which topped a record $70 million last year, cannot be blamed on low staffing levels or union work rules, a fiscal adviser to the county legislature said.

Department brass had cited those reasons for mounting overtime costs, which increased by nearly 50 percent from 2011 through 2016, the Nassau County Legislature’s Office of Budget Review noted in a report released last week.

But the analysis said overtime grew despite changes in the police head count, which dipped in 2012 through 2014, but climbed above benchmark levels in 2015 and 2016. The analysis also found the number of overtime hours attributable to work rules has remained constant since 2009.

The five-page report does not offer a definitive answer about why police overtime increased.

It said the department offered seven different explanations, including a 25 percent spike in sick days taken by police officers in 2016, which accounts for $3 million of the additional costs.

The department also suggested that the additional hours came from policing special events, such as the presidential debate at Hofstra, the Verizon strike and high-profile visits to Nassau by presidential candidates Donald Trump and Hillary Clinton.

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The budget review office noted that it had warned in 2014 that “soft savings” in newly approved labor contracts would have to be achieved to offset “hard expenses” for the county to be able to afford the deals.

“Although that assessment is a few years old, the need for accountability is now more critical than ever,” the report said.

County Executive Edward Mangano said in a statement, “Crime is down 27 percent since 2009, increased security is at its highest demand due to threats to public assemblies and the administration will reduce expenses with the addition of new hires as confirmed by this report.”

The report said Mangano’s consolidation in 2012 of Nassau’s police precincts had saved about $18.5 million through the retirement of 98 sworn officers. But overtime has averaged $19.2 million more annually since 2011.

Also it noted that senior officers earn a larger percentage of overtime than recruits or recent hires, who earn lower salaries.

The finding came a month after a report by County Comptroller George Maragos, an announced Democratic candidate for county executive. Maragos found overtime is “heavily skewed toward a relatively small number of police force members who also tend to be the highest paid and eligible for retirement.”

Budget review said the police department predicted before the consolidation that it could operate just as efficiently without the 98 retiring officers, with a total benchmark head count of 2,275.

The forced dropped to 2,189 members in 2013, but climbed to 2,351 in 2015 and 2,442 in 2016. There are expected to be 2,500 sworn officers in 2016.

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Overtime cost $48.3 million in 2011 compared with $70.6 million last year.