I am going to Western Australia on Working Holiday visa,what sort of Jobs I can do there in farms ????

You name it really. My girlfriend and I have just left a cattle station in the North (The Kimberly region). It's a rough lifestyle, but a real eye opener. When will you be arriving? The season starts in about March or April (depending on the weather). Otherwise there are plenty of farms for wheat, sheep, fruit picking, wineries, all sorts really, and all across the state.
In the North (Pilbara, Kimberly) you will find mainly Cattle stations, though in the far North (Kununurra, Broome) there are also fruit farms. On the cattle stations if you have riding experience (horse or motorbike) you can be a ringer (cowboy/girl) and help with the mustering of the cattle. Otherwise you can get jobs as stationhands (who do all sorts of jobs such as fencing), station mechanics (qualifications not always required), governesses (child minding and educating), station cooks and camp cooks, caretakers or gardeners. There's lots of options, but remember the style of life is harsh. On fruit farms it's generally fruit picking work that you'll find.
Surrounding Perth (for hundreds of kilometres) is the Wheatbelt. You'll find jobs here as tractor drivers, farmhands (same as stationhands), cooks, shearers (or related wool gathering jobs) amongst others.
In the South you'll find wineries, fruit farms, tree farms, deer farms, dairy farms and more. The variety of jobs available here is huge, varying according to what the farm does. There are plenty of jobs available, because the mining industry is booming (and taking all the workers).
If you're looking for a job to extend your working holiday visa, be aware that you can also work in the mining industry. It's more lucrative, but not everyone's cup of tea.
Good luck, I'm sure you'll have a great time