I'm not sure which aspect of the battle this diorama represents but from the shell explosions painted on the background and the denuded, barren ground I suspect that it's the bombardment which ran from 24 to 26 July, described on the Department Of Veterans' Affairs web site:

For three days, 24 to 26 July 1916, the Germans relentlessly bombarded Pozières. The aim of this concentrated shelling was not simply to prepare for a counter–attack but to inflict as much damage and loss on the Australians as possible.