He and his people deny that he punched her.
Mel Gibson: She's Lying and She Secretly Taped Me
According to sources connected with Mel, on January 6, 2010, Mel and Oksana were arguing at home when Oksana took baby Lucia, ran around the backyard, threw herself on the ground, came back inside the house and began shaking Lucia like a ragdoll. Mel followed and the sources acknowledge there was physical contact, but insist Mel did not punch her and was just trying to protect the baby.
His people may well NOW deny that he punched her; HE HIMSELF was recorded agreeing that he did so, and assessing the situation after the fact as her being deserving of a punch to the mouth that broke her teeth while she was holding their baby.