This is a sad reminder that when bushwalking the weather and environment can change rapidly. If you're going to be solo hiking (or even hiking in a group) to bring a personal locator beacon and first aid kit even if you're familiar with the area you're hiking in.
You can hire them for a relatively cheap price from plenty of places and they don't even take up much room! In the past I've hired them from the Blue Mountains Police and Macpac at Town Hall.
Addit: I should also add that it's important to dress for all seasons when hiking. Especially in winter. Dressing in layers and ensuring you have a hard shell layer to stop wind chill and rain/snow.