In terms of the working, I think it can be beneficial to have the job during year 7-11 as you can make some money, learn some skills and there isn't that much studying in those years. However, for year 12 I would probably advise not to do a job during that year because the workload will increase. The last thing you want is to take on a job and then you lose time to it for studying, feel burnt out and due to that you don't get the marks to get into your uni course. I guess though if you're confident you will exceed the ATAR cut off in that case then you can probably manage it.
Once you get to uni though that's a different thing. I would say as soon as you finish Year 12 you should try get a job. There's a lot of jobs you can do aside from having to work in fast food or retail if you don't want to work in those kind of areas, if you have decent fitness and are okay with lifting around 10-20kg parcels, signing up for christmas casuals at Australia Post is a good job to consider. I did that during my 2nd year of uni and it was a fairly easy job in that you don't need to think much, although it is tiring. They also offer like part time jobs later or ask you to come back during other holiday times to people that worked well during that time period. I didn't go back due to getting busy with my tutoring work and uni, but my friend basically worked there the whole of uni with consistent hours and made quite a bit of money. But yeah getting any form of job after Year 12 is very important, because you will begin to gradually build your resume through these lower jobs first and then you can get more specialised later with things relating specifically to your degree once you apply for internships