Paralympian Oscar Pistorius is appealing his recently lengthened prison sentence for the murder of girlfriend Reeva Steenkamp.

Pistorius filed an application to the Constitutional Court on Wednesday morning (AEST), local media report.

The Supreme Court of Appeal (SCA) last month more than doubled Pistorius’ sentence to 13 years, calling the initial six-year sentence “shockingly lenient”.

The earliest the double-amputee runner will be eligible for parole is 2023.

Pistorius is arguing that judgement disregarded material findings of the trial court, which showed “compelling circumstances justifying a departure from the prescribed minimum sentence”.

The usual sentence for murder in South Africa is 15 years.

Pistorius said this was in breach of his constitutional right to a fair trial and sentencing.

Pistorius killed Ms Steenkamp in the pre-dawn hours of Valentine’s Day 2013 after shooting four times through a closed toilet cubicle door with his 9mm pistol.

He claimed he mistook the 29-year-old model and reality TV star for an intruder.

Ms Steenkamp’s family welcomed the longer sentence and said it showed that justice could prevail in South Africa.

When asked about Pistorius’ new appeal, a Steenkamp family spokeswoman ,Tania Koen, said: “It’s his legal right to do so and we have to respect the legal process. It is what it is.”

The SCA last month said Pistorius “displays a lack of remorse, and does not appreciate the gravity of his actions”.

High court judge Thokozile Masipa initially sentenced Pistorius to five years for culpable homicide in 2014.

The sentence was much lower than expected and was widely criticised.

The athlete served only 10 months of the five-year sentence in prison before being released and put under house arrest.

Pistorius earlier this month sustained minor injuries in a fight with another inmate in a South African prison.

“It is alleged that he was involved in an altercation with another inmate over the use of a public phone in the special care unit” where both offenders are detained at the Atteridgeville Correctional Centre,” a spokesman from the department of correctional services said at the time.

Pistorius was taken for a medical check-up after the dispute and was found only to have suffered a bruise, he said.

Pistorius, whose lower legs were amputated when he was a child because of a congenital defect, soared to international fame after becoming the first double amputee to compete in the 2012 Olympic Games.