A person at my school got 48 externally, and got not only nominated but accepted for showcase. This was for a speech though.
They have stuck to 49s and 50s in the past, are now considering 48s that are particularly outstanding in the category.
Of course, internal marks mean something - but only as contributing towards a rank. If you get 25% and everyone else gets 24% you are rank number 1 and it makes no difference if you got 99% and everyone else got 98%. The REAL clincher is the external marks - thats how they see just how valuable your rank 1 is in your cohort! (ahem you will get scaled way, way, down if your cohort does badly no matter what rank you have if your 24% was the case!).
I don't think you could get a 50/50 all up without getting a high rank in your school, however.