A small boy's visit to a suburban Christchurch library led to a court appearance and a $400 fine for his mum.

Mairehau solo mother Maria Rensina Genefaas, 40, dropped off her six-year-old son at the Shirley Library at about 2pm on February 6.

She told him she would be back to pick him up at 4pm.

But she didn't walk him to the door and she didn't check that the library was open.

While Genefaas headed off to do a couple of hours shopping, the boy found out that the suburban library doesn't open on Waitangi Day.

Security staff found him wandering in the area about 40 minutes later, police prosecutor Sergeant Paul Brocas told Judge John Bisphan in Christchurch District Court today.

The boy told police he sometimes went to the library by himself, and his mother arrived at 4pm to pick him up as arranged.

She pleaded guilty today to a charge of leaving a child aged under 14 without reasonable provision for care and supervision.

Defence counsel Vanessa Sugrue said the boy had wanted to go to the library on the last day of his holidays. The boy had been there before by himself because the family had previously lived close by.

Genefaas had daughters aged 19 and 23 and the boy was usually with one of them at all times.

"She is very upset and sorry for what happened and she is grateful that somebody found the little boy and that he's safe," she said.

She was now attending a Best Parenting course in Aranui, run by somebody from the Cholmondeley Children's Home.

Judge Bisphan imposed the fine and court costs, a total of $530.

- NZPA