Vector says more than 300 shareholders sold shares for about two-thirds their market value as a result of an unsolicited offer last month.
The company was one of several whose shareholders were targeted in mass mail outs by Bernard Whimp of Christchurch, who was banned as a company director in 2006.
Chairman Michael Stiassny says such predatory activity has been allowed to go on too long and regulatory agencies must follow Australia's lead and clamp down on undervalued offers.
The Shareholders Association also says urgent regulatory change is needed.
It says anyone who receives an unsolicited offer for their shares should check the current market valuation.
Chairman John Hawkins says people who accepted Mr Whimp's offer not only got less than the market value, but had to wait 21 days for payment.
