Colin Matchim was formally charged with aggravated assault during a brief hearing at provincial court on Thursday. (CBC)

A St. John's man was formally charged with aggravated assault Thursday, in a case involving his four-month-old girl who authorities say sustained serious head injuries.

Colin Matchim, 23, made a brief appearance at provincial court in downtown St. John's on Friday morning, while his former partner, Kate Coombs, watched.

Staff at the Janeway children's hospital called in the Royal Newfoundland Constabulary in March, soon after Coombs brought her daughter, August Matchim, to be treated.

Coombs told CBC News that she left her house to go to the bank and when she returned home found the baby unwell. She then rushed the baby to the hospital.

"When I came home, my baby was really cold and grey in colour," Coombs said in an interview.

Kate Coombs said her baby 'was really cold and grey in colour' befoe she took her to hospital. (CBC)

Physicians at the Janeway found that August had suffered an injury that the RNC later described as "non-accidental head trauma." The baby has since been released from hospital, and Coombs said that she and her child have moved from St. John's to another community.

She said that she ended her one-year relationship with Matchim after her baby was hospitalized.

"I never imagined that he could hurt the baby," Coombs, who is determined to have sole custody of the child, said after Matchim left the courtroom.

"I do not want him having anything else to do with me or the baby. I don't want him having any contact — anything."

The baby is recovering, but is still showing signs of injury.

"We just don't how much brain damage has been sustained by the baby," said Coombs, who added, "Everything is looking positive."

Matchim will next appear in court on April 30.