What is this fruit?

G'day, I'm wondering what this fruit is as I've never come across anything like it before. It looks similar to a small yellow apple, but I found it growing on a low shrub in Canberra. I didn't want to eat any in case it was poisonous, but had a taste and it is very crisp and tart, full of flavour. I've taken photos of the fruit, plant, and the seeds inside. Keen to see if anyone knows what it is?