Does iMessage cost if I'm texting someone in Australia?

I live in the US and my friend lives in Australia. We have have iPhones, will it cost to text over iMessage? Its free only over Wi-Fi right?

What about when I'm not on Wi-Fi, but still texting over Wi-Fi? Will international rates apply then?

No it is free to send/receive iMessages from various countries. Obviously you can send video clips and stills too - all for free.

Use skype.

Its free whether you use celltowers or Wi-Fi, imessage is not part of the text plan. Make sure the option is off in the settings that says send as regular text when imessage unavailable, just so it won't send that way if there's an issue.

Yes, over Wi-Fi it is free.

If not over Wi-Fi then your data plan would be in use. Its not texting since it requires internet, so its I'm.

If you're using someone elses Wi-Fi connection it won't cost you, but if you're using your own the Wi-Fi account will incur data usage charges.