Errol Naidoo, from the Family Policy Institute in Cape Town, was speaking after the 29-year-old South African learned his fate at a Pretoria court on Wednesday morning following the killing of the model and television personality on Valentine's Day 2013.
Oscar Pistorius had originally been convicted of manslaughter before an appeals court upgraded it to murder in 2014.
Judge Thokozile Masipa, who oversaw the original trial which led to the manslaughter conviction, opted not to impose the minimum sentence of 15 years in prison for murder citing exceptional circumstances.
The former gold-medal-winning Paralympian will be eligible to apply for parole in three years, so it is possible he will only serve half his sentence.
Mr Pistorius has never denied shooting and killing Steenkamp but insisted he mistook her for an intruder.
He shot through a bathroom door and says he only realised his mistake when he broke the door down with a cricket bat.
The athlete had previously been sentenced to five years for manslaughter, serving one sixth of the sentence before becoming eligible for house arrest.
Errol Naidoo told Premier's News Hour: "I think it would be fair on the family of Reeva Steenkamp that Oscar get at least 12 years imprisonment.
"It's a serious crime, the murder of a woman, especially in a nation like South Africa where there are high rates of violent crime against women.
"The message we're sending out with this very low sentence of six years is altogether the wrong message.
"It's an insult, it's inappropriate and it should be increased to at least 12 years."
Listen to Premier's Aaron James speaking to Errol Naidoo here:
