WINTER40
Duo sell women's calf fitting boots. This means you can choose boots based both on your foot size, and your calf measurement.
As an average sized woman who is perplexed by the fact that my normal-looking calves often don't fit into standard knee-high boots, I have found Duo boots to be super well fitting. Sizing guides including an instructional video for calf measurement are featured on the website.
Note that the listed prices are VAT inclusive, and the site will remove the VAT when you indicate that you're not from the EU at checkout. As such, for example, the Brindisi boots (http://www.duoboots.com/ladies-boots/brown-nubuck/brindisi/d…) are listed at 170 GBP, but come down to 85 GBP ($170ish AUD) at checkout.
While normally quite expensive, at 40% off minus VAT, these are a reasonable price for the quality.
Currently free international shipping.