Following the capture of Pearl Ridge in December 1944 , the Australian 7th Brigade had been moved south and allocated to the drive towards Buin . In late March and early April 1945 , they had fought the Battle of Slater 's Knoll after which a brief lull followed as the Australians paused to shorten their supply lines . Meanwhile , the survivors of the Japanese force , heavily demoralised by their defeat , withdrew towards the Hongorai River . The Australian 3rd Division was then ordered to resume its advance south , being tasked with capturing the Hari River , while the Hongorai was also included as an " intermediate objective " . Japanese strength in the southern sector was estimated by the Australians at about 10 @,@ 500 men , of which 2 @,@ 300 were believed to be directly opposing the 3rd Division . The 15th Brigade was considered to be the most experienced of the Australian units on Bougainville at the time and was moved up to relieve the 7th Brigade , which was in need of rest .