The private investigator looking for Que Langdon says Kawhia locals heard her father declare he was planning to disappear.
Alan Langdon and his 6-year-old daughter have been missing since they left Kawhia Harbour in Waikato on a small catamaran 19 days ago.
Col Chapman, a child recovery specialist, said Mr Langdon's parents and his ex-wife Ariane Wyler have been adamant he has headed to Australia, and want a search to start there.
Mr Chapman said Kawhia locals had been opening up to him about Mr Langdon's character and his plans with his daughter.
He said one local quoted Mr Langdon as saying: "I am going to take Que, we are going to disappear and no one will know where we are."
It was now feared they could be adrift between New Zealand and Australia, with their small catamaran damaged after bad weather in the Tasman Sea.
Mr Chapman said if Mr Langdon got to Australia, it would be very difficult to locate him.
"It's very, very easy to put ashore on the east coast of Australia. Yes, it is monitored, but it is not heavily monitored. It's not like the northern coast, where they are worried about all the refugees boats coming in. He could easily slip by in a little vessel like that.
"And once he hits the coast, he'll just blend in, like a day cruiser. No one will think that little boat came all the way from New Zealand."
Mr Chapman said Mr Langdon could dock anywhere between the south of Victoria all the way up to the New South Wales Queensland border.
"We will be having meetings with the New Zealand authorities, strongly urging them to take up a request with Australia to treat it as a search or rescue, or as missing at sea. We really do need Australia to pick up the ball here and have a look for them. Once they hit land in Australia, it is going to be very, very difficult to locate them."