The US Coast Guard's icebreaker Polar Star is at Port of Lyttelton on its way to Antarctica.
It will break a channel through the ice of McMurdo Sound so that a cargo ship and a fuel tanker can resupply the scientific bases there.
The Polar Star last called at the port that serves Christchurch in February when the Port Hills were devastated by fire, and on this visit the crew hope to help with replanting efforts.
Captain Michael Davanzo, the commanding officer of Polar Star, says the trip to Antarctica is one of the most difficult US military peacetime missions due to the harsh environment and remoteness.
Antarctica is the coldest, windiest, most inhospitable continent on the globe, and each trip requires careful planning and coordination, he says.
Polar Star is based in Seattle and carries about 150 crew members, 1.5 million gallons of fuel, and enough food stores to last one year in the ice should it be necessary.