Looks like Optus have brought the iPhone SE into their prepaid range.
Not on the Optus website but currently available at Big W for $449 (shows saving of $20 so normal price should be $469, comes with a $50 sim starter kit).
Same cost as the Telstra version but comes with a $50 sim starter kit instead of a $30 sim starter kit.
https://www.ozbargain.com.au/node/301824
Optus website says that prepaid iphones all come unlocked but will wait for someone to test it
http://www.optus.com.au/shop/support/answer/how-to-unlock-yo…
Big W sell the unlocked version for $569 but better off getting this one or the Telstra one where many people have had it unlocked for free.
Combine with Woolworths gift cards from cash rewards for another 5% off
That's a really good price. Purchase with a Woolworths gift card for another 5% off brings the price to $426.55