It's quite beautiful in its own way.
Through a quick Internet search I was able to find that this is a Red Triangle Slug (Triboniophorus graeffei). It is Australia's largest native land slug and grows to a size of up to 14 cm. The distinctive red triangle on its back contains the breathing pore.
The Red Triangle Slug is found along the east-coast of Australia and is most commonly found in heath, woodlands, forests and urban areas. It's diet consists of microscopic algae growing on the surface of smooth-barked eucalypt trees. Apparently, if given the chance, it will also remove bathroom mould. Imagine that. An environmentally safe way of keeping your shower clean - just adopt a Red Triangle Slug as a pet!
In this photo you can clearly see the open breathing pore in the red triangle.
I understand that Red Triangle Slugs come in a variety of colours depending on the location of their habitat. They can also range in colour from pink to bright red to a beautiful mango-yellow colour.