While they returned home, Blong - which means of or belong in Vanuatu Pidgin English - spent time having vet checks and, once approved to leave Vanuatu, flew to New Zealand. He spent 30 days in quarantine and the couple visited regularly, reading to him and taking time to get to know their new feline friend.
"I can't remember exactly how much everything cost but I do know it was more than the holiday," says McMillan, an author of 14 books.
"But I maintain everyone should be allowed one crazy animal incident in their lives. Blong is ours."
That was 12 years ago and now Blong might be starting to pay his own way.
He is one of the inspirations behind a series of children's books which could well propel McMillan's career to the big screen.
The film rights have been optioned for her Elastic Island series about four children who discover a magical island that can "ping" them across the ocean to tropical destinations where adventure ensues. The books — Karen is now working on a third — star a ginger cat called Blong.
It's a series she never intended to write.
"I'd written three books last year and was feeling burnt out," says the author of best-selling novels The Paris of the East and The Paris of the West as well as inspirational books Everyday Strength, Unbreakable Spirit and Love Bytes. "I told everyone I wasn't doing anymore writing because I was going to have a break."
But the summer holidays proved a perfect chance to have some fun and help McKenzie 10-year-old granddaughter, Milla, learn more about writing a book. The characters are partly based on those McMillan created as a child for stories that were serialised in the children's pages of a national newspaper.
She said Milla has made a number of the decisions about how the stories should unfold and what types of adventures and incidents, like falling into a pond and becoming invisible, to include. McMillan recently travelled to Queensland, where she spoke at 15 schools in a week as part of the Australian state's annual children's book week.
"I've always written books for adults so I hadn't experienced what it's like to talk at schools and see thousands of children so enthused about books and reading and with lots and lots of questions for me," she said.
"Writing children's books is so different to writing for adults because you get to go back to being your 10-year-old self and imagining all the adventures you might have liked to have had."
Nor did she expect to feature a ginger cat because there are already so many ginger toms — think Garfield and Puss in Boots — in stories.
"But Blong is such a sweet and unique character, in the end I decided I was okay to write him in. I'm pleased I did as the reaction from children has been amazing — they love him."
