Some union bosses say owning shares in companies in which their members are employed creates a conflict of interest, while others argue it is a good protest strategy.
The Rail and Maritime Transport Union has criticised its Bay of Plenty branch for refusing to sell shares in the Port of Tauranga and Northport, which have been slammed by the union for putting profits ahead of workers' safety.
Engineering, Printing and Manufacturing Union (EPMU) national secretary Bill Newson said his union did not invest directly in companies employing its members.
"The inference that can be drawn is, if you've got a shareholding in a company where you're also representing workers, you've got a conflict of interest in terms of maximising profits and spreading the profits to the workers in terms of better wages."
Mr Newson said the EPMU's share portfolio was managed by brokers, who were overseen by the audit and risk committee.
However, he admitted it was not always possible to know the exact detail of company ownership and this could create some "grey areas" for unions.
Meanwhile, First Union, which represents about 27,000 workers in the finance, retail, transport and manufacturing sector, is currently considering buying shares in some listed companies that employ its members.
General secretary Robert Reid said it could be an effective strategy to ensure workers' rights are represented.
"It's a longstanding tradition of both unions and activist groups to buy small parcels of shareholdings in order that we can have a voice at shareholder meetings."
Mr Reid said this strategy had been used to good effect by anti-apartheid protesters in the 1980s who bought shares in companies investing in South Africa. He said it was still used by First Union's international parent union, which owns a stake in the controversial mining company Rio Tinto.