South African Olympic athlete Oscar Pistorius could serve as little as two years in prison for the murder of his girlfriend Reeva Steenkamp three years ago.

The paralympian was given a six-year sentence by Judge Thokozile Masipa in the high court in Pretoria on Wednesday, after his original five-year conviction of culpable homicide was upgraded to murder on appeal last year. Pistorius, 29, shot Steenkamp four times through a locked bathroom door in February 2013.

Delivering the verdict, Masipa said there was "substantial and compelling circumstances" that mean Pistorius should not serve the 15-year minimum sentence for murder. She said the athlete had to be punished for murdering the model and law student, but added Pistorius had already served 12 months in prison for the initial conviction and he had shown remorse for his actions. It was not clear whether the state would appeal the verdict.

Andrew Fawcett, the paralympian's ' instructing attorney, said Pistorius will serve "between half and two thirds of the sentence" before he can apply for parole.

Yet legal experts have suggested the athlete could spend as little as two years in prison before being released, as he has already served time in jail, undertaken rehabilitation courses and could be eligible for correctional supervision by prison authorities.

South African law states people convicted are eligible to apply for "correctional supervision", which would allow Pistorius to be released early, put under house arrest and electronically tagged, after serving one sixth of their sentence in prison.

Pistorius was handed five years' imprisonment for manslaughter in October 2014. After serving one sixth of the sentence, he was considered for release under correctional supervision by prison authorities and placed under house arrest at his uncle's house in an affluent Pretoria suburb.

Women's rights groups have criticised the six-year sentence as being too lenient, stating it sends out a message of impunity in a country with one of the highest murder rates in the world. Some rights groups have said Pistorius received preferential treatment due to his wealth and status.

Many have taken to social media to express disappointment with the verdict.