"Kosta departs with the very best wishes of everyone at the football club."
Barbarouses, who began his career with the Phoenix in 2007, re-signed with the club last year on a two-year deal from Melbourne Victory, where he scored 19 goals in 81 appearances across all competitions.
After a patchy beginning to the season, Barbarouses found form to finish with five goals from 25 appearances.
He said on Monday the decision to leave was difficult.
"People forget sometimes that there are more factors involved than just football.
"Behind every player are a lot of people making sacrifices - I've been fortunate to enjoy tremendous support, so sometimes you have to think about others and consider their needs."
Barbarouses said he'd come to Wellington wanting to help the Phoenix to an A-League title, and it had been frustrating not to achieve that.
"Wellington Phoenix gave me my first start in professional football.
"There are so many people at this club that I am especially grateful to, and I guess that's something that's made this decision a lot harder."
Dome said it was disappointing to lose a player of Barbarouses' pedigree.
"But every door closing is a door opening, and we remain confident the squad we are currently assembling will have Wellington very much in the mix for finals football."
- AAP