#no funds item not paid

I got paid late this month and my debit orders went through and I got hit with a massive R600 and another R120 on my account the following day. I think I paid it back that same day. This has never happened to me before and I believe should not due to it being a cheque account with overdraft facility?

Do other banks do this? No warning? No info? I am a single mother and cannot afford a chunk of my money to be taken like this? Since moving to fnb I have never have any complaints and have actually recommended fnb to my friends and family but this might just be a deal breaker for me :(