Hi Bill. The best time to visit Australia really depends on where you want to go. At this time of year (our summer) the southern parts of the country are getting temps in the high 20's to mid 30's (and often high 30's)...that's celsius, of course! Occasionally we may get low 40's, but they're not sustained over many days.
On the other hand, if you went to the top end, you would see regular temps in the high 30's to low 40's. A couple of weeks ago, we also saw a temperature of 50+ recorded for the first time since the 1960's! The down side to visiting the northern areas at this time of year is that it is also the "wet season", with lots of tropical storms and cyclones.
Our winters are reasonably wet, but temps are usually around the mid-teens during the day.
For the best general weather experience, I would suggest late March into April and early May, or at the other end of the year, try October, November and early December.