That explains a lot. So what actually happened is that your initial application was approved, but with a credit limit smaller than required for a platinum (which used to be 6k). Then you did a second application that was likely auto-declined by the system, nothing unusual considering you've just had an application for credit approved by the same provider.
I suggest you do 2 things:
1) speak to bankwest and ask if they can reassess you 1st application and approve it with a limit high enough for platinum and with 50k promo attached. They've been known to approve higher limits over the phone and in branch then online, so worth a try.
2) check your credit files with veda and experian to see if the credit enquiries are doubled up, and if there's indeed two hits, speak to bankwest to see if they can remove the duplicate enquiry. This will help your churning game in the long run.