The president of the Rail and Maritime Transport Union is putting pressure on its Bay of Plenty branch to sell its shares in the Port of Tauranga and NorthPort.
Aubrey Wilkinson said holding the shares was hypocritical while the union was also criticising the ports for poor worker safety.
The branch owns the shares but the union has joined a call from unions for a sector wide safety review following five deaths at port companies in the past three years.
Two of the deaths were of contractors at the Port of Tauranga, which also has the highest injury rate.
Mr Wilkinson says the investment is a vote of confidence in the port company.
"I don't support holding the shares at Port of Tauranga. We have had discussions about re-investing them in other portfolios - we just don't seem to be able to get over the line, simply because it's been a good return and it requires very little maintenance."
Mr Wilkinson said it created a conflict of interest that had been robustly debated for years but the Bay of Plenty branch consistently voted to retain the shares.
The branch's organiser, Phil Spanswick, said he personally agreed the shares were a conflict of interest, but pointed out it was a democratic process.
"I think the shares should be divested to other shareholders and we shouldn't be having them but the view at the moment is to keep the shares. It helps the branch very well financially, of course."
Mr Spanswick said individual members had held shares in the port companies, many since they were floated in the 1990s, and the return had been lucrative.
But the general secretary of the Maritime Union said holding the shares was morally wrong.
Joe Fleetwood said it compromised negotiations and his union would never buy shares in an organisation its members worked for.
"Now, if people are dying on the job we deserve and demand the right to run hard at the employer.
"I've got a lot of workers saying how can another union that owns shares, that expects big dividends from that company, now how can it run hard and expose all these atrocities that are happening to workers?"