PHILIPPA YELLAND knew something was wrong when her three children were not waiting at the school gate for her to collect them as usual on a Monday afternoon in mid-March.

And when teachers told her the children had not turned up to their Brisbane school that day, Ms Yelland had only one thought: "Oh my God, he hasn't returned them."

Bokkie, 10, Matilda, 9, and Barney, 7, had spent the weekend with their father, Murray Robertson, at his home in Winbourne Road in the Blue Mountains town of Hazelbrook. Under their parents' divorce settlement, the children visited their father every third weekend.

Ms Yelland has not seen or spoken to her children since she dropped them off at Eagle Junction State School on Friday, March 16, before that weekend with their father.

Ms Yelland, a former Fairfax journalist, has been unable to contact her former husband, who has had his mobile phone switched off since that weekend. "They've disappeared, literally," she said.

In Winbourne Avenue last night, Mr Robertson's former next-door neighbour told the Herald: "He did say once say that if he ever had to disappear he had relatives in South Africa, in the mountains." The man, who asked not to be named, said Mr Robertson was an avid camper and bush-savvy. Mr Robertson had told him that he had found a camping spot "down out the back of Oberon where he thinks no one could ever find him".

Mr Robertson and his children were last seen in the street about six weeks ago.

Another neighbour said Mr Robertson was a member of a "home church" group, in which members worshipped in each other's houses. The woman said a truck came last week to Mr Robertson's home and took away all his things. Last night, a bed and mattresses were still stacked outside the single-storey green weatherboard home.

During access visits Mr Robertson, who did not appear to work, was actively involved in his children's play. The family often put on impromptu theatrical shows in the backyard. Mr Robertson did not allow the children to eat sweets.

Ms Yelland said she had no inkling her former husband would run off with the children. "This was out of the blue, completely."

She pleaded to the public for help in finding her children, who are thought to still be with Mr Robertson, a South African. Speaking via the Herald, she begged her former husband: "Please return the children safely to me. Everyone misses them."

Asked what message she would like to send them, she said: "I love them very much and I look forward to them coming home."

Ms Yelland said she yearned for her children and the day they return home. "I just miss them. Their joyousness, their unending enthusiasm. They are the most glorious children. The place is dead without them."

For now Ms Yelland is taking comfort from prayer. "I believe God will keep them safe. I just pray they will be brought home safely. What else can you do?"

Rather than sitting around worrying, she has been keeping busy, helping family and friends with chores. "There is a great great benefit in constant activity."

The Family Court has authorised the Federal Police, state and territory police forces and the marshal of the Family Court to locate the Robertson children. It has allowed their names and that of their father to be publicised in an effort to find them.

Bokkie is 140 centimetres tall, with straight brown shoulder-length hair, blue eyes, a chipped right front tooth and a spidery red birthmark on her left forearm. Matilda is 135 centimetres tall, with straight blonde shoulder-length hair, blue eyes and freckles. Barney is 130 centimetres tall, with longish white blonde hair and blue eyes.

Mr Robertson, 59, is described as tall, of solid build and weighing about 100 kilograms. He has brown curly hair, blue eyes and scars on his right forearm.

Anyone with information is asked to call the Federal Police or Crimestoppers on 1800 333 000.