The deeply-resented tax, which was hastily introduced by the government last year in a bid to help fill the gaping fiscal hole, has been included in electricity bills.
GENOP Monday said that 1.5 billion bills remain unpaid while another 250 thousand are past due.
Union chief Nikos Fotopoulos on Monday said PPC staff would resist the management's orders for cut-offs, which were planned to begin this week.
The union has in the past disrupted efforts to collect the tax.
Meanwhile, members of PAME, the militant union of Greece's Communist party, on Monday held protests at various PPC offices across the capital.