Bought the 32 GB iPhone SE at the Bourke St Mall Optus Store in Melbourne. Sales staff said phone should be unlocked, and after upgrading to iOS 11, the phone was indeed unlocked (checked with a different SIM). According to this optus page, all new prepaid iPhones from Optus are unlocked.
Likely to be limited stock but at least one other Optus store had iPhone SE's for sale. About the going price on staticice but the all the major stores I checked did not have any SE's in stock.
First deal post for me - hope this helps someone else.
Has the IOS 11 slowed down the phone at all?