McDermott, who had been dating Newton-John for nine years, disappeared in 2005 while on a fishing boat off the coast of San Pedro, California.
In June, a named eyewitness has confirmed to New Idea that the 58-year-old is alive.
When asked if she wondered if McDermott was still alive, Newton-John replied: ‘Ofcourse’.
‘I mean it’s human to wonder. But you know ... those are the things in life you have to accept and let go,’ she said.
‘Because whenever you go through difficult times there’s always those concerns. But you know, I live on, and of course questions come up, that’s human.’
The Grease star has since moved on with John Easterling. The couple married in a private ceremony in 2008.