The father of two missing Australian kids arrives in Christchurch on Friday chasing a hunch they could be secretly living in New Zealand.

Harry Speath last saw his son, Thomas, 6, and daughter, Serena, 7, since just before Christmas 2014, when their mother, Jane Adare, picked them up from his Brisbane home.

Just two weeks earlier, private investigator Col Chapman, said Speath had given Adare, $A350,000 ($376,000) as part of an agreed settlement.

Chapman said the couple had split, but the relationship had been amicable and they had reached a shared custody agreement.

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"They were enjoying shared care up until that moment she disappeared," Chapman said. "They haven't been seen since."

Chapman, who was asked to help with the search six months ago, said despite numerous appeals for sighting through the Australian media and police investigations there hadn't been a single confirmed sighting.

"The kids have not been in contact with any relatives; they aren't enrolled in any school; never been to a doctor or hospital."

He said Speath had for the past two years spent most of his time searching for his two young children; roaming the suburbs, putting up posters in shops and had even gone as far as London and Brazil to try and track them down.

On Friday afternoon, Speath was arriving in Christchurch where it is believed Adare has friends and relatives.

Chapman said authorities also felt it was likely the mother could be hiding in New Zealand, due to its close proximity to Australia and ease of entry.