Mauro Felicori, tasked in August 2015 with transforming the enormous Royal Palace of Caserta near Naples into a hot tourist destination, said on Saturday the complaint had come as "a slap" but he was determined to carry on.
The 18th-century palace in Reggia di Caserta, which boasts 1,200 rooms including a grand theatre, has featured in several films, from the 1999 and 2002 "Star Wars" flicks to "Mission: Impossible III" and "Angels & Demons".
"The trade unionists have to realise that the tide has turned. And the free ride is over!" Renzi said on Facebook, after three unions wrote to Italy's culture ministry to denounce Felicori for working early mornings, evenings and weekends.
Renzi said that with Felicori, 64, at the helm, visitor numbers in February 2016 were up 70 percent on a year earlier, while takings were up 105 percent.
The unions' complaints come amid a crackdown on work-shy public sector workers, after hundreds were caught clocking-in each morning before heading off to engage in more appealing pursuits.
The palace in southern Italy, inspired by the Palace of Versailles and built for the Bourbon king of Naples, was one of the largest buildings constructed in Europe in the 18th century and was designated a World Heritage Site in 1997.
Felicori was one of 20 "super directors" hired last year by the culture ministry to revive Italy's top public museums, including the Uffizi gallery in Florence, Milan's Pinacoteca di Brera and Rome's Galleria Borghese.