If I have a phone on a Vodafone plan in Australia, will it work overseas?

[Stranger in Sydney]

I have no idea if this would work. My mum is gonna get an iphone on a good plan with Vodafone, and she is thinking of giving it to me
I am on Vodafone too but prepaid so it will work in Australia, but I'm going overseas for a few months to Romania and I know that they have Vodafone there too, but will their sim cards work with that phone??

Thanks