Thornley's 100 exactly on Thursday was his third century in the domestic competition this season and followed an unbeaten 261, also against Western Australia, in Sydney last November.
Opener Phil Jaques was the second highest scorer with 50.
Western Australian quick Brad Williams (4-53) used the second new ball to wrap up the tail after bowling largely without luck in the opening session.
Part-time spinner Adam Voges (2-43) also chipped in with the wickets of James Packman for 27 and Nathan Bracken for 16.
Score(day one): New South Wales 305 (D Thornley 100, P Jaques 50; B Williams 4-53, A Voges 2-43) v Western Australia.