The 37-year-old Aucklander took his record to 51 wins from 55 professional fights after a competitive fight against his Nigerian opponent.
Tua was the more aggressive of the two and he frequently had Ahunanya fighting from the ropes.
But the Nigerian proved to be a resilient opponent, taking a number of good Tua punches and returning the odd good blows of his own.
Ahunanya looked close to being knocked out in the 10th round as Tua landed several good blows.
He managed to stay on his feet and continue his record of never having been knocked out, but his lack of aggression meant he was a clear loser on points.
Tonight's victory, following on from his victory last year over Shane Cameron, means Tua can continue on his attempt to secure a second world heavyweight championship fight.
