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Some of the best part-time jobs for students include:

Tutoring
Average salary– $13-25 per hour Bookkeeping
Average salary– $15-20 per hour Brand Promotion
Average salary– $20-25 per hour Virtual Recruiting
Average salary– $20-30 per hour

You can get the entire list of some high-paying part-time jobs here: Part-time jobs for students

The only setback here is that you might have to work odd hours on weekends and holiday seasons.

The job of a cleaner is one of the highest paying part-time jobs in Australia

First of all, do you have a vehicle?
If yes, work as a delivery person.
If not, you can go for Cleaning.
But there are a lot of other high paying part time jobs in Australia that pay a lot more

Professors or other faculty in the specific field are usually the ones who do the hiring for GA positions-

The application process usually occurs a semester before the position begins, and students are hired based on their academic achievements, application materials, and faculty recommendations. The first step for preparation is research. Go to the specific college department’s website to find out what professors are actively doing in their fields. If you can find any information on the graduate assistantships the department offers, read through that too. And use it during the interview.

You can try for one of these part-time jobs for college students-

Tutoring - $13-25 per hour Bookkeeping - $15-20 per hour Data Entry - $10-15 per hour Freelance Writing - $20-25 per hour, Up to $50 per article

For more options, read this article

Dog walking might pay you more
If you decide to work in a relatively expensive area

A virtual recruiter is a liaison between an employer and a potential employee.
They contact several professionals and set up/conduct initial interviews between organizations and candidates.

Read more about it here.

You will really have to know about the history of the place
It's important to be well-read about it

See, that also depends on how much time you have, right
If you have more time and can accommodate a lot of freelance projects...that will earn you a lot
If you have less time, teach a limited number of students and charge a bit more as you get more and more credibility

You can work part-time for 20 hours a week on-campus or off-campus. And you don’t need a work visa to be able to do that! A student permit (aka student visa) is enough.
Read about the details related to studying and working in Canada here.