I have been with Happy House Sitters for almost two years, and have obtained numerous house-sits.
The website is easy to navigate for both Sitters and Home-owners, and I have only had a couple of occasions where I've actually needed to contact them directly, but when I have they have always been courteous, helpful, and efficient.
In my experience, Happy House Sitters is the best service of this kind within Australia, and I would recommend them to anyone interested in house-sitting.