They are reported to have died one after the other by falling into a sewer manhole and succumbing to toxic fumes, sources said Monday.
They were not in a wine tank as was initially reported.
The men were carrying out the work on behalf of Amap, the company that manages the water and sewage pipelines in Palermo.
After the first worker remained underground without coming out, the others climbed down to see what was happening.
The seventh member of the team, not seeing his colleagues come out, raised the alarm.
Four of the victims were Quadrifoglio workers from Partinico, among them the owner of the company; the fifth was a temporary worker from Amap.
Italy is enduring a spate of fatal workplace accidents.
Workplace accident insurance agency INAIL said last week that almost 200 workers had lost their lives in accidents this year, 2.6 per cent down on last.
President Mattarella said in a May Day speech that every such death was unacceptable, while unions called for urgent action to stem the fatal tide.
The spate included seven workers killed in a hydro power plant blast near Bologna last month.
ANSA
