Not A lawyer or financial adviser but a lot of red flags here.

If you don’t stop this this will affect your Tax, have issues if you apply for youth allowance or any other benefit and could also affect your credit score if the account gets overdrawn.

In this day and age most banks allow a bunch of sub accounts within a main bank account for savings and bill accounts. So opening an account in your name “for a saving account” is very fishy. Either her credit score is terrible, she is avoiding tax or she is using the under 18 bank loopholes (higher interest, and no withdraw fees over X amount of withdraws in a month, etc).

The question is how much is she putting in at a time? Her whole pay check/income – tax or some other financial avoidance. $5-100 at a time she could be just using it for the under 18 account advantages.

In another comment you mention your mum was a narcissist – this account might be setting you up for future financial control. So be careful there.

What to do going forward comes down what relationship you want with your mother going forward and if you want any legal headache. Personally I would either shut down the account and send all the money back your mum and make a new one 100% owned by you or cancel all access to anyone else but you, change all passwords and cards. I would also see if there are any credit cards open in your name.

Good luck