Robert and Elizabeth Makara were killed when their SUV hit a tree on Sunday
The husband and wife were rushed to Sunshine Coast University Hospital
Mr Makara, 69, died a short time later and his wife, 68, passed away at 10pm
Local police said officers discovered a suicide note in the car and at a home
But Mr Makara's brother said he might have had a heart attack at the wheel
Anyone with dashcam footage of the crash have been urged to contact police