Tonga's 30-12 opening-round victory over England put them on course for the World Cup semi-finals, and Keaon Koloamatangi said he was proud of their performance.
The 28-year-old represented Tonga 10 times, including at the 2022 World Cup, before switching to Australia ahead of last year's Ashes tour.
He will play prop against Fiji at Suncorp Stadium on Sunday night and join St George Illawarra from South Sydney next season.
"I think if Australia comes calling, it's not something you can turn your back on."
Koloamatangi said he wanted to experience a World Cup in Australian colours, without treating his choice as a rejection of his Tongan heritage.
"I'm just as Australian as I am Tongan."
He called the decision "a tough decision", but said Tonga coach Kristian Woolf accepted it respectfully.
"He was respectful and accepted my decision. I have no regrets," Koloamatangi said.
Australia and Tonga could meet in a semi-final or final. Koloamatangi said he would support Tonga if Australia did not win.
"Obviously, I want Australia to win, but if we weren't to win, I'll be cheering the Tongans all the way. I wish nothing but the best for them."