Lyttelton Port Company confirmed there had been a death on Monday morning aboard the ETG Aquarius berthed at Cashin Quay.
Acting chief executive Kirstie Gardener expressed her "deep sadness" for the worker's family and colleagues.
"Our focus right now is on supporting our staff and the family of our LPC team member at this tragic time.
"At the moment we can't confirm any further details on the incident."
More details would be released later, she said.
In a statement, police confirmed the death and said emergency services were called to Cashin Quay shortly before 9.30am.
Maritime New Zealand has been notified.
"Police will conduct inquiries in relation to the death on behalf of the Coroner."
It is the second fatality at one of the country's ports in a week.
Atiroa Tuaiti died on Tuesday last week while working on a container ship at Ports of Auckland.
His employer, Wallace Investments Limited general manager Felix van Aalst, said in a statement on the day of the tragedy that it was too early to know complete details but confirmed "it was a fall from height".
The 26-year-old's death has led to calls for an inquiry into health and safety at the port.
At the time, Maritime Union national secretary Craig Harrison told RNZ the incident was a tragedy and reinforced the dangers port workers faced every day.
Harrison called for a national inquiry that should be overseen by the union and the Government.
Port workers were overrepresented in death statistics in comparison to other industries such as construction, he said.
