Toronto Police say a recent string of high profile cases has contributed to some $6 million in unexpected premium pay to its officers.

The ongoing investigations into alleged serial killer Bruce McArthur and the suspected murders of Barry and Honey Sherman have "significantly contributed" to the budget shortfall, according to police spokesperson Meaghan Grey.

A report on the service's budget variances will be presented to the police board on Thursday.

Toronto police have been under pressure to freeze spending since the force's budget topped $1 billion. This year's budget flat-lined spending.

Major resources for McArthur, Sherman investigations

Investigators have described the McArthur investigation as the largest forensic operation in the force's history.

Earlier this week, forensic teams wrapped up a nearly-four month search of McArthur's apartment.

Police are also in the process of using cadaver dogs to search dozens of properties around Toronto connected to McArthur.

The homicide squad has also been heavily taxed during its investigation into the deaths of the billionaire Shermans. The bodies of the prominent philanthropists were discovered in the couple's longtime home in December 2017.

Police believe they were killed by multiple suspects in a targeted double homicide.

The two investigations have contributed more than $1 million to the city's $6-million deficit in premium pay, which includes overtime, according to chief administrative officer Tony Veneziano.

Considering savings in other areas, including regular officer salaries, Veneziano says the total budget shortfall amounts to $3.8 million, and he expects to find more savings.

"We don't foresee a deficit come year end," he said.

Budget does not include van attack

The latest budget figures are calculated to March 31, meaning they do not include any additional spending caused by the April van attack on Yonge Street that killed 10 people.

The financial impact of the April van attack that killed 10 people has not yet been calculated. (Aaron Vincent Elkaim/Canadian Press)

The report, signed by chief Mark Saunders, says police will attempt to reduce its premium pay spending in line with its 2018 budget, which was set at $996.3 million by city council.

However, Saunders notes that may not be possible.

"Premium pay is subject to the exigencies of policing and the aforementioned pressures as well as continued police presence required at special events will make this difficult to achieve," the report reads.

'We're seeing morale tank,' union says

The Toronto Police Association (TPA) says an ongoing staffing shortage is the true culprit behind the extra spending. The situation has simply been made worse by the recent string of intensive investigations.

According to the TPA, the service has lost 500 officers over the last five years, meaning others are frequently forced to work long hours.

Toronto Police Association President Mike McCormack called the shortfall "a component of a broader system issue.” (CBC)

"I'm not shocked at all," says Mike McCormack, TPA president. "This is the new normal."

While the extra spending means more money in the pocket of some, McCormack says too many officers are being overworked, which could compromise the quality of police investigations.

"It's not about more overtime, there has to be a work-life balance in there and we're not achieving that," he said. "We're seeing morale tank."

The situation may continue to deteriorate as the service expects to lose 200 officers this year, and it has plans to hire just 140 new recruits.