if I was you, I'd be getting onto nab just to make sure it is...

if I was you, I'd be getting onto nab just to make sure it is just marketing, to ensure it isn't someone else trying to be fraudulent (it probably isn't anything dodgy since you received the notification, but it doesn't hurt to be careful-)
ps is it usual for pre approved personal loans which haven't been sought by customer? I haven't heard of that before.