Winter is cool but not really cold, and it starts from June and lasts till August. So, you can decide your trip according to your preferences. However it is best to visit Australia during autumn or spring when the weather is pretty good. It is neither too hot nor too cold, so you will be able to roam around the valley and check out some of the vineyards with ease and comfort.
Summer is very hot in Australia and it starts from the month of December and lasts till February.