If you are not Australian, the likelihood of getting your data subpoena'd from an Australian company is pretty low. If you're in the US, it's far more likely for your Gmail data to be successfully subpoena'd.
Unless you belong to a fairly narrow set of people, the risk of government access to your data is relatively low. Meanwhile, the threat of advertiser access to your data is high for everyone: Don't trust an ad company with your data, even if they currently say they won't use it for ads.