Alan Langdon had made previous threats about fleeing with his 6-year-old daughter Que, the child's mother says.
Langdon and Que have been missing since they left Kawhia Harbour in Waikato on a small catamaran on December 17.
Ariane Wyler spoke to Radio New Zealand (RNZ) from Switzerland, and said she believed the pair were headed to Australia.
Wyler said Langdon had threatened in the past to disappear with their daughter and she would never see them again.
"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," she told RNZ.
She said although Langdon was a competent sailor, she 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 6-year-old girl across the Tasman in a 6m catamaran."
New Zealand police called off their search for the pair on Tuesday, but say it remains a missing persons inquiry.
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."
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."