1942: First arrival of US Marines
Four months after the Japanese bombed Darwin in February 1942, 20,000 US Marines arrived in Wellington. They entered the harbour on 14 June 1942 on the battered USS Wakefield. A band was waiting for them on King's Wharf, playing the Marine Corps hymn, 'From the halls of Montezuma, to the shores of Tripoli...', as they were the famous marines of the 1st Corps Division.
Though most Marines were in transit to or from combat zones in the central Pacific, they would have been available to defend New Zealand.
From the New Zealand perspective the Americans strengthened New Zealand's defences against possible Japanese attack; while the Americans saw New Zealand as a valuable source of supply and a staging post for operations against the Japanese in the Pacific.
The Marines were mainly housed in military camps in Paekakariki, Trentham and the Wairarapa, with smaller camps and hospital facilities in Wellington city.
Though tensions occasionally ran high between local men and the visiting Americans (leading in one case to the infamous 'Battle of Manners Street' riot in 1943), many life-long relationships were forged.
Arrival US Forces in New Zealand - NZ History Online