Recent changes in World Rugby's eligibility laws allowing players to make an international switch after 36 months of inactivity mean Folau, who played the last of his 73 Tests for the Wallabies in November 2018, is eligible again.
Folau's parents are from Tonga.
Head coach of Tonga Toutai Kefu — a Wallabies legend — has revealed Folau is a well-liked member of the national team's squad and is "just one of the boys".
"The boys love him and, no matter who you are, this mob will bring you down to earth very quickly," he told The Sydney Morning Herald.
It comes after Folau made a rare public appearance in an interview with the Tongan team's social media channels.
"It's been awesome to be in camp with the Ikale Tahi," Folau told Tonga Rugby Union earlier this week.
"For me, to come back to the team, with the opportunity to connect with the boys, back with the culture, and also a big part of that is our faith in God.
"It's been a great week so far, and we're looking forward to spending time with the boys over the next five weeks."
Former All Blacks trio Malakai Fekitoa, Charles Piutau and Augustine Pulu are also expected to be named in the team to face Fiji when it is announced this week.
It is another step towards the possibility of Folau potentially playing against the Wallabies at next year's World Cup in France — a very small possibility with the team's only able to meet at the semi-final stage.
Following the Pacific Nations campaign, Tonga will move onto a World Cup qualifier against either South Korea or Hong Kong with the match to be played on the Sunshine Coast on July 23.
It will still be a shocking sight for Rugby Australia just to see Folau on the field next year.
Folau took the governing body to court for unfair dismissal, demanding $14 million in compensation under employment legislation that bans sacking someone because of their religious beliefs.
The case was settled out of court with Rugby Australia reported to have paid Folau $4 million and the player claiming he had been "vindicated".
