Nkuna was charged with 12 counts, including four counts of attempted murder, trespassing in a national park and possession of an illegal firearm. He was sentenced in the Skukuza Regional Court on Friday.
KNP managing executive Glenn Phillips commended the courts for imposing "such a stiff jail term".
"This is a huge victory and deterrence factor in the ongoing anti-poaching campaign. We warmly welcome this ruling as it will provide the much-needed boost to the anti-poaching teams who endure harsh conditions for the protection of our parks."
He added: "Our wish is to see a significant increase in such convictions and for the law enforcement authorities in affected countries to cooperate and work with us to end this scourge."