Now I can't delete this comment and I realise you are asking about how to say this in German (!) Sorry!! And for that I have no idea. I have just realised you're Australian - you could probably tell me if it's the same in Australian English. Need to switch my brain on this morning!

It's "Eine Freundin von mir" if she's just a friend and "Meine Freundin" if she's your girlfriend.

In British English, we normally just say 'friend', regardless of whether your friend is male or female - it's not really considered important - so usually other people don't necessarily know if your friend is male or female - if it's important then we just ask. We only use 'girlfriend' when we mean that we are in a romantic relationship with someone. I think this might be different in American English. If you want to be clear from the start then you can say 'my female friend', but this sounds quite strange in British English and people are likely to think there's a specific reason you're mentioning their gender. E.g. you want to talk about the differences in opinion between your female friends and your male friends on some issue.