The sea battle which most influenced the war in Papua was the Battle of the Coral Sea, fought on 5-8 May 1942, two months before the Japanese landed there...
The Battle of the Coral Sea resulted from an attempt by the Japanese to transport troops by sea from Rabaul directly to Port Moresby. While the main battle was decided further east, where the United States Navy and Imperial Japanese Navy carrier aircraft attacked each others' carriers, a combined Royal Australian Navy/United States Navy squadron under the Australian-born Rear Admiral John Crace RN, including HMAS Australia and Hobart, was tasked to prevent the Japanese fleet passing through the Jomard Passage just east of Milne Bay. The squadron came under intense air attack but was not damaged.
Coral Sea cost the Japanese a small carrier while the Americans lost a large fleet carrier. The Japanese, unwilling to risk the possibility of their vulnerable transport ships coming under air attack, returned to Rabaul.
Coral Sea determined that there would be a Japanese attempt to attack Port Moresby along the Kokoda track. Had the Japanese been soundly defeated at Coral Sea they would probably have dropped their plan to take Port Moresby. Had they been victorious they would have landed their army at Port Moresby and there would have been no Kokoda track fighting. The drawn battle decided the Japanese to make another attempt, this time by crossing the Owen Stanley Mountains.
Contemporaneously with the Papuan campaign, to the east the Americans and Japanese fought for control of the Solomon Islands. This led to seven sea battles for control of the waters around Guadalcanal Island. In the first of these HMAS Canberra was sunk.
On the east coast of Australia, Japanese submarines, led by submarine ace Captain Matsumura Kanji, sank 14 ships, some of which were bound for Port Moresby.
Along the Papuan coast the RAN carried troops and escorted transports crammed with supplies to support the war on land. Off Port Moresby on 29 August 1942, MV Malaita was torpedoed by the Japanese submarine RO 33. The destroyer HMAS Arunta, Malaita's escort, attacked and sank RO 33. In ten weeks up to the end of the Battle of Milne Bay Australian destroyers and corvettes, particularly HMAS Swan, Warrego and Colac, escorted 36 supply ships to Milne Bay. One of these, the SS Anshun, was sunk there by Imperial Japanese Navy ships Tenryu and Arashi on the night of 6 September 1942.
Towards the end of the fighting in Papua the Royal Australian Navy, in conjunction with the Australian New Guinea Administrative Unit, also maintained a supply route from Milne Bay to Oro Bay south of Buna. In November and December 1942 they brought forward the artillery and tanks without which the Allied victory of Buna-Gona would not have been possible.