SYDNEY (AFP) - An Australian television network on Monday defended its decision to air footage showing Oscar Pistorius in a shock re-enactment of the night he shot his girlfriend, insisting it was not obtained illegally.

The Seven Network broadcast the video on Sunday on the eve of the resumption of the Paralympic gold medallist's trial in South Africa for the murder of Reeva Steenkamp.

The video was made by the Evidence Room, a US company that specialises in forensic animation, and lawyers for the athlete said it was commissioned by his defence team and obtained illegally by the TV station.

The broadcaster disputed this in a statement.

"This was a significant investigation ... examining the critical 85 minutes on Valentine's Night 2013 when Reeva Steenkamp was shot dead by Oscar Pistorius," said Mark Llewellyn, executive producer of the Sunday Night programme.

"The material shown on Sunday Night goes to the heart of both the prosecution and defence cases, including the account provided by Oscar Pistorius.

"We would not have run the footage if we thought we had obtained it illegally.

"The story was run in Australia only and not made available to any other territory."

It did not say how it obtained the footage, in which the double-amputee is seen crossing a room on his bare stumps, wearing a tank top and tight black shorts, his hand clenched in the air as if ready to fire a gun.

The 27-year-old is also seen screaming and crying for help, and carrying his younger sister Aimee down a flight of stairs, as he acts out his account of the minutes after he shot Steenkamp.

Legal observers say it is now unlikely that the prosecution will be able to introduce the video as evidence when the trial resumes on Monday.

Stephen Tuson, an associate law professor at the University of the Witwatersrand in Johannesburg, said the origin of the footage was important.

"If it was produced by the defence as part of their investigation and preparation for the trial, it's strictly privileged, it's confidential and it cannot be used," said Tuson.

"Whatever you tell your attorney is strictly confidential and privileged, if there's a breach of that, there can be a mistrial," said Tuson.

"If there is a failure of justice, that's the end of the trial."

Pistorius, known as the "Blade Runner" for his prosthetic limbs, has been charged with murdering Steenkamp after a row early on the morning of February 14, 2013.

The sprinter claims he mistakenly shot the 29-year-old model and law graduate through a locked door, believing she was an intruder in his upmarket Pretoria home.

If found guilty of premeditated murder, he faces up to 25 years in jail and an abrupt end to his glittering sports career.