I have a free Dropbox account but I have been charged AUD$184.67 by Dropbox on my credit card in the last couple of days.
Please tell me how to contact Dropbox to get a refund on the transaction.
You'll need to contact Dropbox Support directly. Visit the Support page while you're NOT signed in to a Dropbox account, including these forums, and you'll see an option for Billing issues. It's best to use an Incognito or Private browsing session to make sure you're not signed in.
Dropbox cannot charge you if they don't have your billing information. If you've been charged and didn't give them your billing information, then someone else has and you should be contacting your bank.