Baldock has run Monument since 2007 and its predecessor Moncoa since 1998.
Zhai has been CFO since 2001, under Baldock, who will stay on as executive chairman.
The company also hired Mark Braghieri as general manager of operations.
Zhai said Braghieri's experience would help the company restart the Burnakura gold mine in Western Australia and finish the Selinsing sulphide project.
Monument produced 3,400 ounces of gold from tailings in the September 2017 quarter.