Phoebe was last spotted on May 15, but she did not get on a plane from Bundaberg to Perth.
Their residence in Gin Gin has been marked as a crime scene, and authorities have taken possession of a grey Hyundai.
Investigators are asking for any information or sightings of a grey Hyundai ix35 in the wider Gin Gin vicinity from May 15 to 18.
“I know that this investigation isn’t over. It will not be over for me until we find Pheobe,” Johnson said.
“I still hold hope that Pheobe will come home, but I have to consider the possibility that she also won’t.
“If the worst-case scenario has happened, I at least need to know where she’s resting. I need to know where Pheobe is.
“My daughter wouldn’t just disappear. Someone knows something and as a mum I’m asking you to come forward with your information.”
Police say they continue to “run out several lines of enquiry”.
The greater Gin Gin area remains the focus of the investigation.
