Although there are some outbreaks almost every year, in recent years they have not been a huge problem.
There are hardly any serious cases reported from the southern states.
The best protection for travelers is to use a good repelant such as Rid or Bushmans, both are very effective and better than most of the other repelants that travelers bring with them from their home countries.
I have been travelling here extensively for over 30 years and although not imune to mosquitoes, have never contracted either of those illnesses.
Take care,
Mike
--- Live and let live, Peace and happy travels to All. redcentre.com.au