Mayor Bloomberg is expected to announce during his budget presentation today that the city will reserve $2 billion over the next two years to help pay municipal retirees’ health-care benefits.

“Buoyed by one-time tax revenue that exceeds forecasts, [now] is the right time to invest in our future,” an administration official said.

Aides wouldn’t say how much of a surplus the city is running, but it’s likely a tidy sum, considering the hefty amount being set aside. Bloomberg’s total budget is expected to be in the $50 billion range.