Mayor de Blasio’s plan to lift the minimum wage for 50,000 city workers to $15 an hour “establishes troubling patterns” of giving away taxpayer money with no offsetting savings or productivity gains, a watchdog group warned Wednesday.

“Unforeseen or changing circumstances may require reopening contracts, but such occasions should be limited and the negotiations should not be a one-way street,” said the Citizens Budget Commission.

The unilateral raises — which climb each year from as low as $11.50 to $15 per hour in 2018 — cover 20,000 city workers and 30,000 workers at nonprofits with city contracts. The cost was put at $238 million through fiscal 2020, including $120 million per year once fully phased in.

In response, de Blasio said the city can afford the hikes that, he says, will boost productivity even without specific targets.