I don't know if this is the right subreddit and I couldn't find a better suited one where questions actually get answers so if there's a better one to post this on let me know.
I live in New Zealand working part time at a grocery store. The hours in my contract are 25.5 but my contract says I have to pick up extra shifts when needed “within reason”, but my contract does not state what “within reason” is. I have been working this job for a little over a month and a half and since my first week my manager has asked me to come in on almost every one of my days off. It makes it so I almost can't enjoy my days off because I have to mentally prepare myself to come in or deal with the guilt of saying no. I obviously don't come in every time she asks but I do pick up one extra shift almost every week, but I always feel guilty for not coming in more even though the extra shifts I pick up put me at nearly 35 hours which I consider full time (NZ law does not specify how many hours are full time but looks like 35-40 is the general consensus) and full time isn't something I can emotionally or physically handle right now. I feel bad because lately several people in our department have quit/been ill and I know we're pretty short staffed, but I feel like that isn't my responsibility especially because I DO come in as much as I can handle.
So my question is, what is considered reasonable amount of extra shifts? Can any employees with similar contracts or (unbiased) managers give any input? I feel like an extra shift almost every week is a lot considering our shifts are long (I work 3 days a week but 2 shifts are 9.5 hours and one is 8 hours, sometimes when I'm called in I work 10 hour shifts, and I have to spend 1+ hour on the bus to/from work or spend money on an Uber on the weekends when my normal buses aren't running) and it almost puts me at full time even though I'm contracted as a part time worker. She even asks when I've already picked up an extra shift that week. It's exhausting especially because it's a gross job (I work in the seafood department where I scale and gut fish for customers, along with a lot of minor physical labor, that sounds whiney but I struggle a lot with my mental health and have some physical issues that make me a lot more sore and tired than I should be working a job like this). Thanks for reading.