Dempsey, a 21 year old free transfer from Sheffield United made 29 appearances for the Cobblers in defence will depart for Australia where he hopes to continue his career.
Meanwhile Lopes a player who came through the Sheffield United youth programme has not featured at all this season, making a total of 6 appearances in the time spent at Sixfields.
Kevan Broadhurst said:"I wish both players every success for the future and would like to thank them for their efforts here.
"In Paul's case, this is a terrific opportunity for him and I hope it works out.
"Both players have found chances hard to come by but hopefully they will have benefited from their time here."