Ann: NAB Chairman and Group CEO arrangements, page-6

Yes, I know he resigned and obviously he had a 26 week notice for termination / resignation in his contract.
I doubt very much that Thorburn suddenly had an epiphany and decided it best for all that he should go and also not give the agreed notice. The "resignation" would have come after some "discussions" including a time frame and termination agreement. The Company obviously felt it better to pay him out rather than have the circus carry on for a further 26 weeks.
By the way, a long notice requirement in these sort of contracts is not unusual as it often takes some time to find a replacement and it can be quite damaging if a very senior person moves on suddenly. (Very easy to argue against this practise, but that's how it is)
The board would have had little option and I think it best they just pay him and move on, even if it sticks in the craw...………..​