I work two jobs, both are casual, and I am claiming the tax free threshold from one job. Each time I am paid from the other job (the one I am not claiming the tax free threshold from), my employer withholds a different amount of tax per working day, anywhere between $80 and $125 per day. I am wondering how my employer determines how much tax to withhold each pay period since it seems to be done completely randomly.
Likely the answer is : They do what they are told. Most employers do not run employee tax calculations - they hire a company to do it. Unless you are really large, doing those calculations yourself is stupid - there are liability issues that an external firm has to handle via their insurance. Also there possibly are a lot of regulations and updates, so unless you are in the business of writing this software or providing this service, you are just running a minefield of things you can do wrong.
The more detailed answer is: they follow the law. Likely it depends only on your income of the day, with some extras in case. There are rules (not even guidelines) for this, and the software that is used (no one does that by hand) just follows them.
They actually are published, i.e. https://www.ato.gov.au/calculators-and-tools/tax-withheld-calculator/
Do you work for an hourly wage, or a salary? I'm betting it's an hourly wage.
For each paycheck, they have to assume that's what you make every pay period (because the payroll system isn't smart enough to know how much you're going to earn for the whole year and use that tax rate).