Australian troops of the 30th Infantry Battalion took Madang and Alexishafen on 26 April 1944, finishing the long pursuit down from Shaggy Ridge. The next day an American destroyer anchored in Bostrem Bay, and the US Navy's Seventh Amphibious Force began eyeing the sheltered water as a forward repair base. Formal construction started on 13 June 1944 when 200 men of the 91st Naval Construction Battalion came ashore with lumber and heavy equipment, transferred from Naval Base Finschhafen and Naval Base Milne Bay. They built on the Alexishafen south peninsula and on two small offshore islands, Megas and Ulimal. The whole facility was complete on 17 August 1944, just sixty-five days after the Seabees landed. Speed was the entire point.

For a few months in 1944, Sek Harbor held one of the strangest gatherings of ships in the Pacific War: floating workshops parked in what had been a German Catholic mission harbor. The destroyer tender USS Rigel (AD-13) rode at anchor. The repair ships USS Achilles (ARL-41) and USS Remus (ARL-40) tended to the landing craft and motor launches flowing in from the Rai Coast. USS Culebra Island (ARG-7) handled engine repairs. USS Midas (ARB-5) worked on battle-damaged hulls. The two floating drydocks sat in Sek Harbor, ready to lift small warships out of the water for hull inspection. PT boats, including PT-341, used the place as a forward base for night patrols against Japanese barge traffic still running along the north coast. A Seabee camp, supply depot, machine shops, ammunition dump, and fuel tank farm completed the picture.

The Allies liberated Hollandia on 22 April 1944, four days before Alexishafen was even taken. By the time the base finished construction, the fighting had jumped hundreds of miles west, and then jumped again to Biak and beyond. This was the logic of MacArthur's island-hopping campaign: bases were built to serve an advance that was already leaving them behind. By November 1944 Alexishafen was too far in the rear to matter. Parts of the base were struck and shipped forward. On 25 December 1944, eighty men of the 91st Construction Battalion began packing up the most useful gear to send to new bases closer to the action. The US Navy formally closed Alexishafen on 28 January 1945. The freshwater supply system, built to serve thousands of men and now needed by no one, was transferred to the Royal Australian Navy.

Very little physical evidence of Naval Base Alexishafen survives in any conventional sense. The drydocks were towed away. The ships returned to general duty. The Seabees went home or to the next base. What remained was absorbed back into the bush, into the older layers: the Catholic mission that had given the place its German name, the Australian administrative network that followed it, the villages along Bostrem Bay that had always been there. Alexishafen today is one of many small mission stations dotting the coast north of Madang, best known for its church architecture and its wartime airfield, and occasionally for a visitor who arrives looking for traces of a floating American repair yard that was operational for a little over six months. The shape of the harbor still holds it, even if nothing else does.