Partner pining for his child.

My partner has a 2 year old daughter whom he hasn't seen in over a year because her mother refuses him visitation. My partner pays $600+ a fortnight in child support and she just denies him access. At first, when he was allowed to see her, it was only for a few hours a day (they broke up before the baby was born), then it dwindled to an hour or so every few days, then the excuses started coming.
"I have to go pick the boys up from school, you have to leave". And when he would say he would stay with the little one more excuses, until one day she just refused access altogether. I know he is devastated even though he keeps a strong front in front of myself and my little girl but I know it eats him up inside. When he was in between jobs recently he missed 2 child support payments, undertandably, and for the first time in almost a year she contacted him, just to abuse him for not paying her, even though she lives very comfortably off of Centrelink (3 children, full care), the other child support payments from her other children's father (good paying job, so payments are quite large) and the fact she doesn't pay rent as she still lives with her mother. He still buys her presents every chance he gets but after then being either sent back or thrown out he no longer tries to give them to her, he keeps them instead for the day he can give them to her without them being discarded and to show he he never forgot about her no matter what her mother said. His mother does the same, she's her first grandchild and she is also devastated. He wants to start the process of mediation but doesn't know where to start. I don't know what I'm actually asking but it baffles me as to why women do this without cause? He is an amazing father, he has never ever put the child or her mother in danger nor made them feel unsafe so how can this keep happening! where do we start? Who do we contact? Thanks.