The house was smoke-logged but not badly damaged.
One of his children reportedly alerted the babysitter on finding a bedroom was full of smoke and everyone evacuated the apartment, the Brisbane Times reported.
Safely outside, Mehrtens was joined by Gregan as they watched fire crews bring the blaze under control.
Gregan then offered his home to his old rugby adversary until his family found somewhere else to live, said the Brisbane Times.
Mehrtens was reportedly grateful for the offer, and relieved that everyone escaped unharmed.
The All Black great recently relocated to Sydney with his wife and children after finishing his rugby career in France.