There may be a minimal negative impact in the changeover in your organic traffic but that would only be short term, as long as you follow Google's guidelines of how to move your site from one domain to another.
Long term, a .com.au should help your audience better determine that you are an Australian business, and you should get some localised benefit from using .com.au because Google has always had a bias towards domain names. However that has become less prevalent in recent years as Google looks at a range of things to determine whether a website is localised or not, such as the regional language code used on the website (ie. en-au), and to what you set the geographical audience to in Search Console, and of course, it still comes down to the actual content of the site. If they all point the same direction then the localisation benefit will be strong.
If you haven't already, you will need to setup a Google Search Console account (previously known as Webmaster Tools).
For the future, I would redirect the .com to the .com.au until such time as you need a international web presence, and then have either have a separate site for international or if you want separate regional websites, roll them all into one under the .com and serve regionalised websites (ie. /au and /gb /fr /us etc.) under that.