From the Department Of Veterans' Affairs web site:
The autumn rains had set in by the time the Australians reached the Somme and the whole battlefield had become a sea of mud. Broken ground, easily traversed in dry weather, was a bog. Trenches and tracks were often impassable. It could take relays of stretcher-bearers many hours to bring in a wounded man, the mud slowing the journey to a kilometre an hour.
On 18 November 1916, the Battle of the Somme officially ended and for the remainder of the winter of 1916–17 the Australians garrisoned the line east of Flers. From there they kept pressure on the Germans by means of small attacks and raids. However, the main battle was against mud, rain and frost-bite.