US ice-cutting ship Polar Star helps out in Christchurch en route to Antarctica

A United States Coast Guard ship has arrived in Christchurch, on its way to complete an ice-cutting mission in Antarctica.

The Polar Star crew will reach Antarctica on January 9, but before they leave they're lending a hand in Christchurch.

The journey hasn't all been smooth sailing.

"We've had a lot of holding on for the last day and a half, so everybody was really happy when we got into the harbour here," says commanding officer Captain Michael Davanzo.

The 121-metre-long ship is heading to Antarctica to cut a channel through the ice, clearing a pathway for US supply ships to get to the National Science Foundation base.

Breaking through a 25-kilometre stretch of ice is no mean feat.

"If you know the little rumble strips on the highway, when you get on there you kind of bounce," says Capt Davanzo. "If you can imagine that 24 hours a day, it's like that all the time."

The work is tough on the 41-year-old ship, with a team of extra engineers brought in to keep close watch on its condition.

The crew work from a vantage point 42 metres up - five flights of ladders above the main deck.

"It gives us a good field of view and lets us plan out our route better, so that we take the easiest path through the ice," says communications officer Kyle Smith.

For this mission, a Royal New Zealand Navy member will be on board too.

"He's just trying to garner as much information from them in terms of how they do it, to see if it will actually suit the way we do business down there," says Christchurch resident naval officer Lieutenant Commander Andrew Hogg.

Before they leave for the month-long Antarctic mission, the crew are using some of their downtime to help out.

The last time the Polar Star was in Christchurch, the US Navy lent a hand with the Port Hills fires. While they're here this time, they'll assist the fire service in replanting some of the destroyed vegetation.

The ship and her crew will leave Lyttelton on Tuesday, bound for the ice.