Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about how to resurface and fix your bathtub.
I'm sorry to hear that your plumber has damaged your bath. I recommend using the PC-11 56g White Epoxy Paste it's waterproof, non-toxic, and once dry can be sanded down. I suggest mixing a small batch and feathering the edges when applying it to the damage. Please note that you will not get an exact colour match as the epoxy will be off white when it dries. I suggest using White Knight 500ml White Tub And Basin Paint to go over the repair.
If you need more advice or information, please let us know.