On 1 July, the Battle of the Somme commenced. This mainly British offensive involved attacks against German trench systems on either side of the Somme. British troops pushed slowly forward towards Bapaume until they were just short of the village of Pozières, at which time the Australian 1st Division entered the fray in the Battle of Pozières. The German positions at Pozières were known as the "Old German" (OG) trenches – OG 1 and OG 2. The Australians launched a preliminary operation on 22 July, but were repulsed. The following day, the division attacked again, this time with the 1st Brigade on the left, and the 3rd Brigade on the right. Leak's battalion was allocated a 550-yard (500 m) sector on the far right flank of the divisional attack. The troops captured the first objective, but were held up by a German position where OG 1 met "Pozières Trench".
The 9th Battalion was being held up by a pair of German machine guns. An intense bomb (hand grenade) fight began, during which the heavier Mills bombs used by the Australians were outranged by the lighter German Model 17 grenades. Leak ran forward and threw three Mills bombs into the machine gun post, then leapt into the post, attacking the garrison with his bayonet. By the time the rest of his platoon got to the post, Leak was wiping blood off his bayonet with his slouch hat. The fighting around Pozières continued after this incident, and two days later the Germans laid down a tremendous artillery barrage on the positions that had been captured from them, described by the official Australian war correspondent, Charles Bean, as "among the heaviest that occurred either on the Somme or at Verdun". During the fighting and the subsequent deluge of shells, the 9th Battalion had been reduced from a strength of 1,016 men to 623.