Smith, who spent two spells with Bognor Regis Town, pounced to score in the 45th minute to give the hosts a 2-1 lead at the break.
It came after Invicta took a 1-0 lead thanks to Ian Gayle on 10 minutes but the visitors’ leading scorer Tommy-Lee Higgs levelled things up from the penalty spot in the 24th minute.
But it was one way traffic after the break, certainly in terms of goals, as the Kent side muscled their way to victory with Smith grabbing his treble. He scored on 79 minutes to pretty much take the game away from the Rocks and then completed his hat-trick four minutes after the 90.
Bognor Regis Town: 1 Ryan Hall, 2 Spencer Spurway, 3 Hayden Gale, Calvin Davies (C) (Doug Tuck 82′), Amadou Jallow, 6 Tommy Block (Tyler Edmondson 82′), 7 Harvey Whyte (Billie Clark 62′), 8 Matt Burgess, 9 Dan Gifford, Charlie Lambert (Jasper Mather 37′), Tommy-Lee Higgs. Subs: 12 Jasper Mather, 14 Tyler Edmondson, 15 Preston Woolston, 16 Doug Tuck, 17 Billie Clark.