The South African private military contractor who died when his tank was attacked in Nigeria knew what his mission might entail, his wife Almarie told Beeld newspaper.

"He went into it with eyes wide open. Luckily he died instantly, which every soldier would want, instead of suffering," she reportedly said.

Lotz and five Nigerian soldiers were killed when they were fired on at a roadblock near Maiduguri on Monday.

His commander from his days with counter-insurgency unit Koevoet during the border war with Namibia, identified only as "Nangombe", was also reportedly killed.

The South African and Nigerian governments have denied knowledge of South African soldiers training Nigerian soldiers against the militant organisation Boko Haram.

He is the second South African to die there after another man died of a heart attack six weeks ago.

His wife said he had resigned from work in Iraq to go to Nigeria. He was due home on Monday for a rest.

"I have peace in my mind that he did what was in his blood. Everything just feels so unreal," she said.

She said their son Leon, 22, was proud of his father.