The mother of Que Langdon says her ex-partner has repeatedly threatened to take their daughter and disappear forever.
Alan Langdon and six-year-old Que have been missing since they left Kawhia Harbour in Waikato on a small catamaran on 17 December.
Ariane Wyler spoke to RNZ from Switzerland, and said she believed the pair were headed to Australia.
Ms Wyler said she missed Que terribly, but she was determined to remain positive that she would be reunited with her daughter again soon.
She said Alan Langdon was a competent sailor, but questioned his decision to set-off with their daughter.
"I don't think anybody would say anybody that's right in their mind would take their six-year-old girl across the Tasman in a six metre catamaran."
Ms Wyler said she hoped to travel to meet Que as soon as she heard she was found.
New Zealand police called off their search for the pair on Tuesday, but say it remains a missing persons inquiry.
Ms Wyler implored Australian authorities to pick up the search, saying she believed they wouldn't be far off the Australian coast.
"Start searching, start actively searching, put it out in the media.
"If Alan and Que are on the Australian coast, and other sailors see them, it wouldn't come to their mind that they've just sailed across the Tasman
Ms Wyler said Mr Langdon had made threats in the past that he would disappear with their daughter and she would never see them again.
She believed he was capable of doing that.
"Australia is very large and people do disappear, and I totally believe he is capable of doing that."
"Once he gets to the shore he can disappear, he can vanish and I'm convinced of that."
Ms Wyler said she would consider Mr Langdon as a good father, but there were certain things she disagreed with him on.
Ms Wyler said her daughter was 'awesome' and very staunch.
"She's been brought up on a boat... her first four-and-a-half-years of her life, so she's very physically active. She's just gorgeous."