It's used when someone wants to convey hope or encourage that a situation will get better. My car broke down, but she'll be right after a visit to the mechanic.
Pro Tip 2/3
Casual Conversation
Typically used in informal settings among friends or family to reassure someone. I forgot my umbrella at home, but she'll be right -- it might not even rain.
Pro Tip 3/3
Cultural Identity
The phrase reflects a laid-back aspect of Australian and New Zealand culture. We Aussies don't panic easily; she'll be right, mate!