Police have charged a man with assault as a 2-year-old girl fights for her life in hospital.

Police spokesman Kevin Loughlin said a 27-year-old man would appear in the North Shore District Court tomorrow on a charge of assault.

He said more charges would be laid once the full extent of the girl's injuries is known.

The parents of the girl told relatives the 2-year-old had fallen on two separate occasions in the days before she was rushed to hospital.

Police are investigating how Seini Ikamanu, also called Jane, was injured at her home on Ranch Ave in Beach Haven, on Auckland's North Shore.

The youngster, who can speak only Tongan, was taken to Starship Hospital about 8.20pm on Wednesday and has since had surgery. Last night, she was still in a critical condition.

One of the girl's cousins, Mele Moala, yesterday told the Herald the 2-year-old's parents had told her the child had whacked her head on a television cabinet on Monday and then was pushed over by her 1-year-old brother the next day.

But a neighbour said the child had been happily playing with her 2-year-old son about an hour before she was taken to hospital.

Seini had moved back in with her parents - Sela and Kefu Ikamanu - and her little brother in January, after having lived with her grandmother in Tonga since soon after she was born in New Zealand.

Mrs Ikamanu's niece, Mele Moala, was at Starship on Wednesday night when a doctor explained to the family that Seini may not live.

"She's severely brain damaged ... The doctor told us her brain is swollen, they're hoping the swelling will go down, they opened the left side of her skull to relieve some pressure ... but it's possible she may not make it."

Mrs Moala said when a doctor opened Seini's eyelids and shone a torch on her eyes, her pupils did not react.

"The doctor said that usually patients react to light but her eyes didn't even move."

Her aunt had not gone into much detail about what had happened, Mrs Moala said.

"I couldn't get much out of them, they were too upset."

Her aunt was a "really good mum", and Seini was a happy little girl who would "talk away, laugh and play" with Mrs Moala's kids. She had seen the little girl on Sunday and she seemed in good spirits.

The family's house was under police guard yesterday as officers went door-to-door speaking with neighbours.

Detectives examined a set of wooden stairs at the front of the house which led up to a deck and two concrete steps by a garage.

A neighbour, who did not want to be named, said her 2-year-old son often played with Seini and her brother.

On the day she was taken to hospital, the boy had climbed a ladder on his property to get to the top of his slide so he could see the little girl over the fence. She was standing on her deck so she could see him.

"They were chatting and laughing with each other over the fence like they always do," the woman said.