It’s now beyond official: Gross mismanagement and obscene union contracts are the chief culprits driving the MTA’s outrageous overtime costs. That’s the bottom line from a scathing 61-page report released Thursday by outside auditors.

“For years, MTA leadership at all levels has been on notice of management’s failures to address overtime issues but has permitted these failures to persist unabated” nonetheless, the report said.

Since 2014, extra-hour costs have soared 53 percent — to a whopping $1.3 billion last year. In a series this year, The Post shined a spotlight on jaw-dropping OT at the agency’s LIRR division, based on shocking data from the Empire Center.

Some workers clocked more than 3,000 hours of additional pay in a single year.

And “individual high earners of overtime represent only a small proportion of the overtime MTA-wide,” noted the auditors, Morrison & Foerster LLP. They singled out, in particular, problems at the LIRR, citing rules that have “little to no modern justification” — but that pushed the division’s OT last year to $225 million.

LIRR workers get nearly double the OT rates of their counterparts at Metro-North. They can “double, triple and even quadruple” their pay “from a single shift.” Moving an engine earns a worker eight hours’ pay but takes just an hour.

Timekeeping systems need updating, enforcement needs to be improved and, of course, work rules need to be fixed.

Many of these problems have long been known, as Nicole Gelinas notes recently. Still, the attention they’re getting now, following The Post exposés and the Empire Center data dump, makes them impossible to sweep under the rug any longer. It’s time for Gov. Andrew Cuomo and MTA chairman Pat Foye to finally act to fix what they know needs fixing.