A swell small in height but big in force has seen Port Taranaki in lockdown since Saturday.
Port Taranaki harbourmaster John Ireland said the port would reopen on Wednesday morning at the earliest.
A west to northwest swell ranging between 10 and 15 centimetres in height was surging into the port over a 10-second period, he said.
"It's very low in height but it's very long and it causes ships to start moving inside the harbour," Mr Ireland said.
"It's like the whole harbour is heaving."
The long-wave activity began about 11am on Saturday, he said.
There were currently no big ships in port and one was anchored offshore.
Long swell periods could cause mooring lines to snap, he said.
"It is an extreme hazard."
In the past snapped lines had had given stevedores broken limbs and strains and caused damage to infrastructure, he said.
Before swell forecasting technology was employed at the port, between 20 to 30 lines would snap each year.
In the 12 months since the technology had been used no moorings had snapped.
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