South Bunbury will clash with Bunbury in the 2019 South West Football League - Colts Grand Final, on September 22.
On Sunday, the Tigers faced off against Busselton in the preliminary final at Hands Oval.
After claiming a one-point lead at half time, the home side posted four goals to one in the third term.
Jed Montgomery kicked five goals in the Tigers' 11.6 (72) to 5.10 (40) win.
Busselton's Jack Kennedy and Ky Spowart slotted two goals each.
Harvey-Brunswick-Leschenault's Reserves did enough to secure a two-point victory against Eaton.
The Lions claimed a seven-point lead at the first change, before kicking three goals to two in the second quarter.
Both sides rallied to kick three goals each in the third term.
Conceding two goals in the fourth, the Lions held on to clinch a 9.9 (63) to 9.7 (61) win.
Eaton's Joshua Kelly top scored with four goals.
