A man has admitted murdering a Victorian teenager who was bashed to death and dumped in an old mineshaft.

Thomas Kelson, 28, pleaded guilty on Friday to murdering Cayleb Hough, whose body was found in bushland west of Melbourne about three months after he died.

Kelson's plea comes weeks before he was due to stand trial for murder in the Supreme Court.

Cayleb's body was found in the Lerderderg State Park near Macedon, in March 2016.

Prosecutors alleged Kelson bashed the boy for hours in Collingwood, believing he had harmed Kelson's father.