Depends what you're wanting to see. Western Australia is renowned for it's wildflowers, so spring is good - colourful. It's also a great time to look at rural areas (like our farm) as the pastures should be lush and green and the animals fat and happy. Weather is mild (although we only ever get one or two frosts even in winter). Autumn is also nice, although we don't have the deciduous trees that you do, so the colours aren't as spectacular. Autumn is the stillest time of year - I love it - but on the farm, feed is tight and the animals are skinny and grumpy! The calves are bounding around though, so that's kinda cute. Summer is fine as long as you don't mind the occasional hot day (read over 100 F) and winter OK as long as you don't mind rain and drizzle.
In other words, any time is a good time and even if the weather was lousy, we'd make sure you enjoyed your stay here anyway.
