We've shown you how to set your Nexus 5 to use Australian English, but unfortunately the nifty 'OK Google' command to automatically start speech recognition and search for what you say only works with US English. Here's how to have both Australian English on screen and all the 'OK Google' goodness.
The key is that Google actually has separate settings for the on-screen display language and for the voice search language. As long as you set the voice search language to US English, the OK Google command will work, but you won't be stuck with US-centric spellings.
To change that option, swipe down from the top, tap the top right corner and then choose Settings. Scroll down and choose "Language & input". Scroll down and choose "Voice Search". Tap on "Language" and make sure "English (US)" is selected.
Your mileage with the feature will vary; it seems to misinterpret roughly half of everything I say, but a brief test with Scottish-accented Kotaku editor Mark Serrels worked flawlessly. But when it does work, it's very handy.