Free shipping with AU$49 spend on products sold by Amazon AU OR Amazon US/UK. Cannot be combined. 

Must be a recent change because the order I placed on 19 September 2022 had an AU$59 minimum spend for free shipping. 
For eligible items sold by Amazon US or UK, you'll now see Free international delivery if you spend over NZD 56.06 on eligible international orders.

International Free Shipping - Amazon Customer Service

Orders sold and fulfilled by Amazon AU
or sold by a third-party seller and fulfilled by Amazon shipping to New Zealand with at least AUD $49 of eligible items based on the order total inclusive of 10% Australian GST before checkout, qualify for free international shipping excluding Amazon Global Store orders.

For Amazon Global Store orders; 
you must add over AUD $49 of eligible items inclusive of 10% Australian GST before checkout which are dispatched and sold by Amazon US or Amazon UK to qualify for free international shipping.

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