@Addison
I've been making calls to Costa Rica with GV.

Open Gizmo5 and login. Wait for the success sound.
Open web browser and go to https://google.com/voice
Click add credit and add $10 using your credit/debit card.

Australia is $0.02/min to call landline, $0.14/min to call cellphone, and $0.17/min to call satellite phone.

* Click green Call button,
** click the flag and scroll down to Australia (+61). input the local number 4582154XX
** click the phone to call with scrollbox and choose your Gizmo5 number (assuming you have already "paired" it)
* Click connect
Your Gizmo5 application will now ring, I have set mine to auto-answer but it doesn't by default, click the green Accept button that will appear on-screen.
It should now ring/call the person.

If you prefer to use DialCentral over the website just dial 011614582154XX

011 is the US call-out/international code
61 is the Australia international call-in code

If you are unsatisfied with the call quality (my N810's microphone echoes on the other end) and you don't mind using "anytime" minutes on a cellphone...Dial your Google Voice number (doesn't have to be a phone that you have paired), when you get to voicemail then dial * you will be prompted for your PIN number. dial 1234 (or whatever your PIN is, don't press #). Then press 2. Then 011614582154XX#. This will be a local call to a mobile phone (Google) and then Google will take money out of your prepaid account with them to connect you to the international number.

Minutes/amount spent per call shows up in GV history AFTER the call has ended.