Original: After initially being posted to the 1st Battalion, in March, 1916 he was transferred to the 53rd Battalion, which was part of the 5th Division, when the Australian Imperial Force was reinforced with fresh recruits from Australia.
Simplified: After being posted to the 1st Battalion at first, in March, 1916 he was moved to the 53rd Battalion, which was part of the 5th Division, when the Australian Imperial Force was helped by new soldiers from Australia.