The main crossing was planned for 20 May , with the 58th / 59th Infantry Battalion on the right tasked to cut the Buin Road and the Aitara Track to the east of the river , while on the left the 57th / 60th Infantry Battalion would divert the attention of the Japanese off the 24th Infantry Battalion which would make the main frontal assault from the centre of the Australian line , crossing at the Pororei ford , advancing straight up the Buin Road . Preliminary moves began before this , and on 15 May a platoon from the 24th Infantry Battalion along with two tanks attempted to carry out an attack on Egan 's Ridge . After one of the tanks was held up and knocked out by a Japanese field gun , they were forced to withdraw . Meanwhile , the RNZAF Corsair squadrons — now reinforced by No. 16 Squadron — began an eight @-@ day aerial campaign , attacking along the length of the Buin and Commando Roads . During this period , the New Zealanders flew 381 sorties , while artillery and mortars fired " thousands of rounds " .