Left-back Heffernan, 29, has agreed a move until the end of the season from A League side Central Coast Mariners.
His deal is subject to clearance from the Football Association.
England youth international Eccleston signed a new deal with Liverpool earlier this season and made his Premier League debut as a substitute against Fulham in October.
Terriers boss Lee Clark said: "He has real pace and can play anywhere across the front line, whether that is as a striker or on either wing. He is very highly thought of at Liverpool."
Heffernan was capped twice by his country during last year's Asian Cup qualifiers.
He had a season-long loan with German Bundesliga side Nuremberg during the 2006-7 campaign.