Queensland man Michael Watter says he will not stop searching until he finds his twin daughters who have been missing for two years.

Key points: Watter twins believed to be in northern NSW with new identities

Watter twins believed to be in northern NSW with new identities AFP says many children taken overseas

AFP says many children taken overseas Abduction consultant says calls for help are increasing

Sisters Isabella and Bronte Watter and their mother Cassie Watter vanished from Townsville in April 2014.

Their disappearance occurred several months after Mr Watter was granted full custody of the girls.

"It doesn't get any easier in that time and it probably gets a bit worse with each day when you're hoping they'll come back but they don't," he said.

"I really love being a dad and it's a big part of my life that's gone."

Australian Federal Police (AFP) and Queensland detectives are actively investigating the case.

They believe the Watter twins may have been given new identities and they and their mother are most likely being supported by friends.

Michael Watter with his daughters Isabella (left) and Bronte. ( Supplied: Michael Watter )

New leads in northern New South Wales are under investigation.

Last year, a website in support of Cassie Watter published more than a dozen endorsements.

Anyone with information on the Watter twins is encouraged to contact Crime Stoppers.

Mr Watter, who has moved from Townsville to Brisbane in the hopes it will bring him closer to finding his daughters, said his family is offering a $50,000 reward for anyone with information that leads to their return.

Many abducted children taken overseas: AFP

The AFP said many parental abductions ended up overseas, with two to three children a week brought into Australia or taken out of the country by a parent.

"Many people believe that if the child is with someone they trust, they are not in danger and should not be considered as missing — this can be a fatal assumption," an AFP spokesperson said.

"The complexity and seriousness increases when a child is abducted across international borders, which creates a greater emotional trauma and confusion for all parties involved - the child, the family left behind, the abductor and law enforcement."

Former Australian soldier Adam Whittington helps parents track down their children domestically and internationally.

Former soldier Adam Whittington said calls for help from Australian parents were increasing. ( Supplied: Adam Whittington )

He told the ABC there had been an increase in calls for help, especially in Australia.

"Demand for [help finding] Australian children is quite high; it's probably ... where we get most inquiries from," he said.

Mr Whittington said it was the children who suffered the most.

"They go from town to town and many children we find have been locked up in apartments. They [parents] don't go to doctors because they can be traced," he said.

Cairns man Stuart Dempster had his daughter returned from Thailand last year with help from Mr Whittington and his team.

"I was used to having her there all the time and suddenly she was gone; it leaves a huge void in your life," Mr Dempster said.

"There's a lot of things going through your head and it's really difficult to function."

But he said perseverance paid off and urged other parents not to give up hope.

Mr Whittington said parents should also be vigilant for scammers who could try to take advantage of them.

"Having this [abduction investigation] industry regulated would stop these cowboys and idiots who are just completely ripping off these parents," he said.

"These parents would sell their house ... just to get their kids back."