The discovery of a child's shoe at the bottom of an Avoca mine may be the breakthrough investigators need in the search of missing Maryborough boy Terry Floyd.

The shoe, believed to have been worn by Terry on the day he disappeared 42 years ago, has been found wedged underneath tonnes of gravel and cement dumped down the Morning Star Mine at Avoca.

Daryl Floyd has found a shoe during a mine search for his brother's body. . Credit:Lachlan Bence

The disused mine is strongly thought to be the final resting place of 12-year-old Terry, who was believed to be murdered on or around June 28, 1975.

The almost fully intact upper part of a child's shoe has now been given to investigating police for forensic testing.