Ao and Po are the male and female forces in the universe. Ao is light and day, while Po is darkness and night - however, this darkness is not evil, it is good and nurturing as is the earth or the womb.
The first act of creation was the separation of Ao from Po which, when finished, allowed all else to occur. Ku, the Creator, instigated this separation and then drew out Kanaloa the squid, who later became the god of the sea. He then drew out Kane (Tane in New Zealand), and it is possible Kane came from the union of Ku (as Father sky) and Hina (Mother Earth). Kane coupled with a few beings and from the couplings came grass, streams and reptiles. However, he wanted a child in his own image. He took clay and fashioned Hine-hau-ona (earth-formed woman). Their first child was Hine-titama (dawn-woman). But Kane became wicked and took his own daughter for a wife, concealing from her that he had helped form her.
Hina knew that Kane was in the wrong, as it was direct violation of the great "kapu" (taboo) against incest. When Hine found out that Kane was her father, she ran to her mother's domain, the Po world of the dead. Hina cried: "You have broken the umbilical cord of the world!", and so Kane could never touch his daughter again. Hine-titama and her mother became rulers of the dead. Kane was now confined to the world of Ao, above the ground.
Kane's children now reside on earth during their life because of his crime. When they die, they descend to the Po world of the dead, underground, where Po creates a safe barrier between the two worlds. Po in life for us is the "creative world of night", dreams, lovemaking and the appearance of spirits. (Parallel Myths, p.57)