Are there any free textbooks/resources for high school physics online?

Well I'm not really sure about textbooks, but there are a lot of free websites you can use to develop problem solving skills for physics, such as isaacphysics.org and the past papers on the intermediate physics olympiad website.

I don't know how different the Australian Physics syllabus is from the British one, but you might want to look at the GCSE flashcards, notes and past papers on physicsandmathstutor.com. They also have past paper Cambridge and Oxford entrance exam physics questions which may be of use.

I hope this helps!

Oh wow thank you so much! Exactly what I needed.

UmuTech (https://umutech.net) is also a brilliant resource that contains loads and loads of exam questions to practice on. It provided us with loads of practice on a variety of topics. I would definitely recommend checking it out! :)

Hey @Asif Iqbal,

I am going to copy and paste @Sriram A 's answer as the formatting bugged out for him.

BBC Bitesize has very good sets of notes covering each topic, along with some questions.

Mathsmadeeasy and physicsandmathstutor are very good for brief notes, flashcards and past paper questions.

Isaacphysics and past paper olympiad questions are useful for developing problem solving skills.

The WJEC Physics specification is helpful for diagnosing areas for improvement.

Hope this helps!