Isabella and Bronte Watter, then aged seven, were taken into hiding by their mother Cassie Watter on April 4, 2014.

They have not been seen since.

Police said in 2014 that they did not fear for the safety of the children in the care of their mother.

The girls, now aged 11, were with their mother when they were found in Taree, on the morning of Friday, May 4.

The children's grandmother, Heather Doubleday, said she was relieved to hear from her daughter after four years of silence.

"We had a phone call from Cassie saying she was in police custody in Taree,” Mrs Doubleday said.

“She was found with Bronte and Isabella.

“Cassie sounded very composed.

"It was a bit of a shock but it was just lovely to hear from her."

Mrs Doubleday said she and her husband Arthur were preparing to seek custody of the children.

The discovery of the children follows a four-year police search for the missing trio.

The father of the children, Michael Watter, mounted an ongoing media campaign to try to locate the children.

The mother had complained about an unsatisfactory police investigation, and a complaint lodged with the Crime and Misconduct Commission eventually resulted in a finding in March 2016 that the investigation had been 'inadequate' and 'unprofessional'.

As a result of the findings, one officer left the police force and others were retrained.

However, by the time the findings were released Mrs Watter and the children were missing.

Several inquiries have been underway since she and the twins disappeared in 2014.

Friends of the mother and children, who can't be named to protect their identity, have supported Mrs Watter since her disappearance.

National domestic violence helpline: 1800 737 732 or 1800RESPECT. In an emergency call triple-zero.

Australian Associated Press