Signed contract lowering my hours since it wasnt formed as Q

Hi all, looking for some advice on contract issue and feeling exploited as young person working at supermarket

👋Please let me post I deleted first one since it was missing important info since I rushed it 👋

I originally had a contract for 7.5 hours every Sunday, but over time I was consistently rostered less than that (sometimes as low as 4 hours). Later on, my employer gave me a new agreement reducing it to 7 hours, but it wasn’t really presented as a choice — it was more like “you need to sign this.” I was also 17 at the time, so I didn’t feel like I could push back.

My manager said the reduction was due to “budget,” but other coworkers in the same role didn’t have their hours reduced.

They’ve now done an audit and offered me backpay based on 7 hours (around $900), but it doesn’t seem like much given the situation. I also don’t feel like I genuinely accepted the change — I wouldn’t have agreed to reducing my own hours if I thought I had a real choice.

I’m also aware there’s a 90-day timeframe for raising a personal grievance, so I’m unsure how that affects things given this has been ongoing for a while.

So what i want it to be backpaid 7.5 hours NOT 7, and to have my hours restored to 7.5 as the original contract had but this is trixky since ive been working under 7hr contract for over a year and havent had courage to say i didnt really agree to it since I feared retaliation.

I know this is a complex issue so pls read properly, and I have a legal background so not after simple advice

Thanks heaps!!🙏