Star Paralympian Oscar Pistorius returned to the witness box at his murder trial on Tuesday, insisting he and girlfriend Reeva Steenkamp had a loving relationship and saw a future together before he shot her dead on Valentine's Day.

Pistorius sat before a packed courtroom wearing a dark grey suit describing how the couple met and gradually began to plan their future, a day after an emotional first day on the stand that saw him issue a tearful apology to the victim's family.

The Paralympic gold medallist is accused of premeditated murder over Steenkamp's murder in February 2013, but claims he shot her through a locked toilet door believing she was an intruder.

On Tuesday the athlete known as the "Blade Runner" described how he met the model at a track day car event.

"The first six days we knew each other we called each other every day," he said.

"I was very keen on Reeva," he said. "I think if anything I was more into her, than she was at times with me."

"We started really seeing a future with each other," he said.

Called as witness in his own defence, Pistorius has insisted he and the 29-year-old law graduate, model and aspiring actress had a loving relationship and sought to explain text messages in which Steenkamp said she was scared of him.

"I just think it was a bad hour in our relationship," he said, describing a fight at an engagement party.

Steenkamp had sent Pistorius a message saying: "I'm scared of you sometimes and how you snap at me and how you will act towards me."

Steenkamp's mother June attended Tuesday's hearing along with Steenkamp's friend Gina Myers, who was wearing tags that bore a black-and-white print of the model.

On Monday Pistorius had made a tearful apology to Steenkamp's family.

"I can promise that when she went to bed that night she felt loved," he said choking back tears, his voice faltering.

"There isn't a moment, or hasn't been a moment since this tragedy happened that I haven't thought about your family."

Fast cars and guns

Pistorius's initial evidence has focused on countering the prosecution's portrayal of him as reckless and obsessed with fast cars and guns.

He has spoken of a difficult childhood, exposure to crime and challenges of his disability.

He is likely to remain on the stand most of the week as his extensive testimony is expected to be following by gruelling and likely lengthy cross-examination.

Besides answering a charge of murder, Pistorius also has to testify about three unrelated charges: two of firing a gun in separate incidents and one for the possession of illegal ammunition, found in a safe at his home.

Cape Town-based criminal advocate William Booth said much will depend on how Pistorius stands up to cross-examination.

"Oscar's evidence can only be properly assessed once he's been cross-examined," Booth said.

In the five weeks since the trial began, Pistorius has appeared fragile and sometimes annoyed, frequently crying in court.

He was physically sick when the gruesome details of Steenkamp's death were discussed.

A key question will be, "Is he emotionally stable to testify if he's breaking down all the time?" Booth said.

"One would rather have him in a better state of mind when he testifies."

Pistorius has hired an extensive team of forensic experts, including an American animation firm that will visually depict the crime scene using three-dimensional computer-generated images.

The experts will have to cast doubt on the state's version of events, including testimony from witnesses who said they heard a woman screaming on the night of the killing, which would show Pistorius knew his target was Steenkamp.

His lawyer Barry Roux said he will call 14 to 17 witnesses in his case to testify on ballistics, urine emptying, damage to the toilet door, sound, and "disability and vulnerability."

The case continues to grip South Africa despite going on much longer than its original length of three weeks.

Newspapers feature front-pages of Pistorius almost every day, and radio talk shows invite callers to speculate on the athlete's innocence or guilt.

Prominent figures in the case, notably defence lawyer Barry Roux, have become household names and spawned Internet memes.

Crowds of people queue outside the courthouse, jostling between the line of international media to get a picture of Pistorius.

The trial is slated to run to at least mid-May.