Hi, Astrid, sorry that I have just only seen your post.
I live a few hours north of Sydney and the one thing I wanted to say is about the size of Australia. It can be snowing in Tasmania and you would fry in Cairns or Darwin. I have also suffered through sleet in NZ in December.
About clothing - I would go for comfort over fashion every time. If those shoes work for you then take them. Definitely take flip flops as we wear them all the time. Everywhere. It is generally warm enough in a lot of places to continue to wear them in winter. I like the idea of the thin rain jacket as it will protect you from wind as well as rain. I don't own any wool but then again I don't hike.
About jeans. I probably don't wear jeans from about now until March as it is too hot and humid unless I am guaranteed of somewhere with frigid air conditioning. But for NZ or hiking they would be an essential. I would take a light pair of 3/4 pants or whatever you normally wear. Also shorts as you can wear shorts just about anywhere here.
A fold-up hat, sunglasses, swimsuit and beach cover up should just about do it.
Have fun!