Google takes in many signals when it comes to generating Seller Ratings, including the review feeds that come in from the review partners they work with (including ProductReview.com.au). By default, ProductReview.com.au provides a feed nightly to Google.
However, during feed processing, Google uses certain algorithms to ensure only what they feel are high-quality reviews are used for Seller Ratings. While they might receive more than 150 reviews for a particular merchant within 12 months, not every review is eligible to be shown.
Like their search algorithms, Google is quite secretive about how their manage reviews in Seller Ratings. Since this whole process is based on proprietary algorithms, we don't have the insight to share what makes certain reviews appear over others.
The best practice we can provide is that while we are still relatively new to the Seller Ratings program, as more time passes by more of our reviews are becoming visible. For brands where their reviews are coming almost exclusively from ProductReview, we are seeing them appear and we also continue to monitor how often we appear through search queries.
For even more information on Google's Seller Ratings, please read this helpful article here.