This is going to be a long post so heads up! My best friend is pursuing astrophysics so I'll give you the advice he has given me for this post.
First of all, at Unimelb under the bachelor of science, both the mathematical physics and the physics major can lead to further studies in astrophysics. However, I would strongly advise doing the physics major as there are some subjects you can take where you can do "astrophysics research". I'll list some subjects at unimelb where you can study astro-related content:
- From the solar system to the cosmos (PHYC10008) [1st Year]
- Special Relativity and Electromagnetism (PHYC200015) [2nd Year]. Note the person currently teaching this subject is an astrophysicist so he uses a lot of astrophysics examples to explain the content
- Lab and Comp 3 (PHYC30021). You can ask to do an astrophysics research project for the whole semester
- Astrophysics (PHYC30019). Self-explanatory; completely astro.
- Science Research Project/Science Internship (SCIE30001/SCIE30002): The opportunity to work in the astrophysics department and can be counted as two subjects.
-EDIT: Whoops forgot to include Archeoastronomy (PHYC20017) and Indigenous Astronomy (PHYC10010)!
In a Unimelb physics major, compulsory subjects to graduate are Lab and Comp 3 and Quantum Physics whereas mathematical physics requires Quantum Physics and Complex Analysis. Both majors are fine, but for astrophysics, a physics major is desirable as applying for postgrad studies, you would qualify for both theoretical and experimental physics. (It's hard to build up your experimental skills for astro if you do a mathematical physics major)
To be very honest here, if you plan to go down the astrophysicist pathway, I would strongly recommend going to ANU to do honours in physics, specialising in astrophysics as that is the uni that gets the biggest pool of data for the astro department. I wouldn't say a bachelor's degree there is worth it, but their honours in astro definitely is. Monash also has an astrophysics major, however, I have heard from friends that have transferred that it's not the best... (would stay clear). I also decided not to go to Monash for physics as their physics department is not very strong in terms of research.
If you are looking to stay in Victoria, I would recommend Unimelb with a physics major such that you can use your other electives to gain an astrophysics internship and research experience. I'm also in a similar position where I solely want to do Experimental particle physics and nothing more haha. No matter which uni you go to, there is going to be a lot of maths you can't escape and general physics knowledge you will need to know before pursuing the higher-end physics areas (like astro). A uni that has a strong research group is usually where you want to go anywhere physics related! I've also heard the University of Sydney is pretty good in the astro area if you are thinking of moving interstate.
I personally would say doing a general major in physics would be more desirable than doing a major in astrophysics just in case you change your mind later down the track. A physics major will look better if you are looking to apply for an honours/masters as you probably would have taken more general "core" physics subjects like quantum physics, electrodynamics, statistical physics and experimental work. However if you did do an astrophysics major, so long as you take the core physics subjects you should be fine.
TLDR; A bachelor degree anyway should be alright, just so long as you have taken the general core physics subjects. Taking an honours/masters would definitely be needed if you plan to go down the astrophysicist pathway. In this case, honours at ANU for astrophysics (1st preference by far) or if you want to stay in Victoria, a masters at Unimelb should set you up for a research pathway.
EDIT: Should also mention that Swinburne has a massive astrophysics/astronomy department which is something to consider if you don't want to relocate. Would still put ANU above that in terms of quality though.
If you have any questions about physics, feel free to reply to the thread!
Hope that helps!