Now we know why the unions fought so hard to keep it a secret: The Empire Center for Public Policy reports that FDNY pensions averaged $100,000 for new retirees these past seven years.

That’s a huge jump over the years, in good part because most firefighters who’ve retired since 2001 got disability pensions, which pay 75 percent of final average salary, tax-free — and under rules that presume heart and lung ailments are service-related.

Two-thirds of all FDNY retirees now get disability pensions — 73 percent of those who retired from 2002 to 2013. And, as the Empire Center notes, the rise in disability pensions began long before 9/11 — so “Ground Zero syndrome” doesn’t explain it away.

After a long legal battle, the Empire Center got the state’s top court to rule that taxpayers have a right to see both the names of pensioners and the amounts they get.

It’s no trivial stuff: As the Empire Center’s E.J. McMahon recently noted on these pages, the city’s pension costs will hit $8.8 billion in the coming fiscal year — more than twice the 2006 level and nearly eight times the 2001 amount. It’s one of the top burdens on city taxpayers — and growing.

And the firefighters pension system’s assets are just 54 percent of what it needs to cover liabilities, so that burden is sure to grow.

One limit: Disability pensions for those hired since 2009 pay just 50 percent, with an offset of Social Security payments — and without that presumption.

But the unions, backed by Gov. Cuomo, want to restore the old, more lucrative disability rules for firefighters hired after 2009. The bill would soon reach an extra $100 million a year — and keep rising.

Mayor de Blasio is willing to raise the benefit — but only for those who qualify for Social Security disability: no automatic presumption. That’s hardly unreasonable.

The firefighters’ union notes that the biggest pensions are going to retired managers and chiefs. Fair enough — but that hardly makes the problem go away.

The Post has uncovered too many outrages — like FDNY Lt. John McLaughlin (a k a “Johnny Lungs”), who won an $86,000 disability pension for bronchial ailments, then competed in marathons and triathlons.

We fully support fair treatment of New York’s Bravest, especially those injured on the job. But (for once) the mayor has it right here: “Our city needs a pension system that doesn’t unfairly burden taxpayers.”

© 2015 New York Post