The Rail & Maritime Transport Union is calling for a review of safety systems at Lyttelton Port following two deaths of port workers in the last few weeks.
Police were called to the port just after midday on Saturday after a man died on board a docked fertiliser vessel.
The previous death was of a contractor in a logging truck accident on 26 November.
The union has added its concerns to those of the Maritime Union of New Zealand, which has called for a meeting to discuss health and safety at the port.
The Rail & Maritime Transport Union says safety at Lyttelton Port needs serious attention following the two deaths.
Its general secretary Wayne Butson says its sympathy goes to the family of the worker whose death will be heart wrenching for them just before Christmas.
He says the Lyttelton Port needs to work with unions to review its systems following the Christchurch earthquakes, as it is now congested, with too little space available.
Maritime Union of New Zealand national secretary Joe Fleetwood says the watersider who died on Saturday was a union member, and both fatalities have shaken port workers.
Mr Fleetwood says the port has become more dangerous since the 2011 earthquakes as it has grown busier.
"There's not enough storage, not enough land down there for the volume that they're taking in."
Mr Fleetwood says the union is calling for a meeting of industry workers early in the New Year and if concerns over health and safety at the port are raised they will need to be addressed.