The tell all: Oscar Pistorius' attempt to explain the unimaginable.

In an interview on South African channel ITV, broadcast in Australia via 60 Minutes, Oscar Pistorius talks about killing his girlfriend Reeva Steenkamp. 

Speaking to interviewer Mark Williams-Thomas, Pistorius repeats the familiar story about mistaking Ms. Steenkamp for an intruder.

He tells of lying down to sleep beside Reeva in a humid room where the air conditioning unit was broken, and turning on the fan to keep cool.

He asked Reeva to close the doors and switch off the television, and they both fell asleep.

Then, the unimaginable happened.

Half asleep, Pistorius realised that Reeva had forgotten to close the doors. He got up to close them, leaving Reeva in bed and his prosthetic legs behind.

Then, Pistorius heard a noise coming from the bathroom.

“This instant fear comes over me that there’s somebody in the house,” he says of his first thought.

“Somebody’s was actually in the process of breaking in. And my first thing was thinking that I need to grab my firearm. If this person’s already got into the house, it’s a matter of seconds before they’re in the bedroom.”

He claims he whispered to Reeva, who he believed was still in bed, to "call the police and get down on the floor."

Knowing he couldn't make a run for it without his legs, his only thought was to get between the intruder and Reeva.

"At least I'm between them and her," he recalls thinking.

"I'm starting to shake, sweating. I start screaming, 'Get out of my house. Get the fuck out of my house.' At that moment, I hear the toilet door slam."

"I don't have time to get my legs back on, these people are in the house already. If I hesitate, they could use the gun on either of us."

He shouted again for Reeva to call the police.

"I'm petrified at this point. I don't have the balance to align the firearm."

Then, he noticed that the bathroom window was open, confirming his fears of a break-in.

"I can't go back to Reeva and expose her and expose myself. All I know if there's somebody in my house, and all of a sudden I hear a noise at the toilet.

"Before I knew that I had fired,  I had fired four shots."

Not really knowing what was happening, Pistorius made the decision to go back and check on Reeva.

"I get to the bed... and I get this chill over me. I look back and I see Reeva isn't in the bed. I start hitting the bed and I go, 'Reeva, Reeva, Reeva', and then a sense of calm came over me because I realised, she listened to me, she got down on the floor.

And I get down on the floor and I can't feel her so I start pulling everything apart. I'm putting my hand across the curtains thinking, 'Lord, please just tell me she's hiding behind the curtains.

"I'm still scared there's an intruder in the house but now it's mixed with another fear.

"So I rush on my stumps as quick as I can to the bathroom, it's locked, and I realise there's somebody in there. So I run to the room, and I'm trying to put on my legs as quickly as possible."

Pistorius tried to charge down the door, but was unsuccessful.

"I started screaming 'Please, please please please, just don't let this be what I think it is.'"

He ran back to his room to grab his cricket bat and smashed down the door.

Reeva was on the floor behind it.

"When I saw Reeva, she'd slumped over the toilet. And I thought, at that point, I knew that I'd killed her, I knew that she was dead. I went down on my knees and I pulled her onto me.

"I turned so that I could get her out of the toilet. I put a towel under her head. There was just blood everywhere. So much blood."

Pistorius says he didn't know what to do after picking Reeva up. He imagined that Reeva has started breathing so tried to give her mouth to mouth, but there was too much blood.

Watch the trailer for tonight's 60 Minutes here.

"You could ask yourself a million times, 'Why didn't I just close the door behind I went to bed? Why didn't she tell me when she got up?' She didn't do anything wrong, but it's difficult. It's difficult to know if one of those small things didn't happen..."

For his part, Pistorius is haunted by the events of that night.

"I can smell the blood. I can feel the warmness of it on my hands," he says.

"And to know that that's your fault, that that's what you've done. And I understand the pain people feel, that loved her and miss her. I feel that same pain. I feel that same hate for myself.

"I don't argue with anyone who wants to lock me up and throw the key away... but if the premise is I killed Reeva intentionally, I do not agree."