Wellington's third all-time highest goal scorer asked to be released from his short-term contract once it became clear the finals were out if reach.
Phoenix came from a goal down to draw 1-1 with Sydney on Saturday but it wasn't enough after Perth Glory sealed the final top-six spot with a 3-1 victory over Brisbane Roar later that night.
“The opportunity has happened quickly as can happen in football,” Smeltz said.
“I was totally focused on the Phoenix and had we made the playoffs I would have stayed till we finished playing."
Smeltz has made 12 appearances since joining the club in December on a deal until the end of this campaign.
His most recent goal came in the 89th minute of Wellington's stunning 3-0 away win over Melbourne Victory last week.
“At my age I can’t afford to slow down," the 35-year-old added.
"And with the All Whites’ World Cup qualifiers and the Confederations Cup on the horizon I need to keep the good level of fitness I have built up at the Phoenix," Smeltz added.
“I have spoken to (All White coach) Anthony Hudson and he is happy with what I am doing.”
Smeltz’s has had a much travelled career, first joining the Phoenix in 2007 before returning for his second stint this season.
Between both spells the prolific goal scorer had stints with Gold Coast United, Shandong Luneng (China), Genclerbirligi SK (Turkey), Perth Glory, Sydney FC and Kedah FA (Malaysia).
Smeltz flew home to Australia on Sunday afternoon before heading to his new club.
Phoenix coach Des Buckingham described the A-League hitman as a true professional.
“Shane arrived at a time when we were at the bottom of the table and struggling,” Buckingham said.
“From the moment he arrived he applied himself as a top professional on and off the field and has been part of the much improved performances in the second half of the season.
“I wish him all the best in his new venture which I’m sure will be successful.”
Smeltz scored 24 goals in 51 appearances for Phoenix and is the A-League’s all-time second top goalscorer, with 92 goals in 190 appearances.