The 22-year-old half-sister of three young children who have been missing for nine months says the family desperately need the public's help to find their loved ones.

"There's only so much the police can do, we really need the public's help," she said.

The half-sister — who asked not to be named — spoke to the Herald after police yesterday made a nationwide appeal for sightings of Otorohanga dad Tom Phillips and his children Jayda, Maverick and Ember.

"They are just kids, they haven't made these decisions themselves. It's not like Tom said to them 'hey do you mind if we leave everyone you know and go into hiding?" the woman said.

"It's so, so wrong."

She said their disappearance had been hard on her and her mum.

Since they had been gone, she said they had missed Ember's 6th birthday, Jayda's 9th birthday, Christmas and her own birthday.

The three young children, Jayda, Ember and Maverick Phillips have been missing for more than five months. Photo / Supplied

"I used to call them at least every month and come up to visit once or twice a year but it's nine months and I haven't spoken to them once," said the half-sister, who lives in Timaru.

"Honestly, I feel like he's told them I'm dead or that I don't want to talk to them because they would not go this long without wanting to talk to me.

"We used to go to the beach together and I took them to Marokopa falls," she said.

Oranga Tamariki "closed the file" on the missing Waikato children in May and was not seeking to check on their welfare, she said.

"Just this month they reached out to me and my family to try and form a plan to when the children return. We haven't had the meeting yet."

Phillips and his children have been the subject of aerial, bush and shoreline searches in the Waikato region since he failed to reappear in the Te Kuiti District Court on January 12 on a charge of making a false statement.

He and the children were officially reported as missing by family members on January 18.

In late January, police were made aware his vehicle, a ute, had been parked on Mangatoa Rd near the Mangatoa Track.

Police said yesterday the children's disappearance had caused their mother and family "extreme stress and anxiety".

Waikato West Area Commander Inspector Will Loughrin urged anyone with information about the family's possible location to come forward; that included even if they knew of a new family group in their community — "specifically a father and three school-aged children".

Waikato West Area Commander Inspector Will Loughrin initially said Tom Phillips had not violated any court restrictions and done nothing wrong in that regard. Photo / Mike Scott

"We are mindful the photos we have made public to date may now not accurately depict what Tom and the children look like," Loughrin said.

"When Tom Phillips was last seen by family in February, he had a beard and that alone will make him look considerably different than in the photos we have circulated."

The children's appearance will have also changed since they were last seen by family in January.

"They will be taller; they will have either lost baby teeth or started to get their adult teeth and will likely be wearing their hair in different styles."

They could also be using different names, Loughrin said.

"If you think you may have seen them, or know where they are, we want the opportunity to follow up that information."

A map of the area in which Phillips' car was found when he first disappeared for 17 days in September last year. Image / File

Police continue to have regular contact with their extended family, with Loughrin adding: "I want to acknowledge both the children's mother and Tom's wider family who continue to be living with extreme stress and anxiety."

Loughrin said police also wanted to hear from anybody around the country who had seen a man and three school-aged children travelling inter-city on trains, buses or ferries, knew of three school-aged children in their community who didn't appear to be attending local schools, or had experienced the theft of petrol, diesel, food or camping supplies.

Phillips' three children are; Ember Nirvana Essence Phillips, 6, Maverick Rusty Callam Phillips, 7, and Jayda Jorga Jin Phillips, 9.

- NZ Herald