"I've spoken to him a few times and his brother as well - he's very determined to play for Tonga so we're very happy about that," he said.
Kefu said Piutau will be joining a club that had the interests of Pacific Island players at heart. Former Blues coach Pat Lam had a strong Pacific influence on his playing roster.
"Pat Lam really understands our plight and understands that these Pacific Islander players are very keen to play for their country whenever possible."
Winger Halai is playing in France for Pau after transferring from Wasps. He had one test for the All Blacks in 2013.
Kefu said Halai was very interested in playing at the World Cup for Tonga.
"There's also some ex Wallabies - Sitaleki Timani and also Anthony Fainga'a - there's a few more as well but that's just the few names that scratch the surface."
Players who have represented their country's top or second 15-a-side team or senior sevens team can switch allegiance to another nation, provided they have a passport for the second country and have completed three-year stand down.
The first time a player represents their new country must be in an Olympic event, which can include Olympic qualifying tournaments and the 2018/19 World Series.
But Tonga are not on the main World Sevens circuit. ''We would have to come through the Oceania sevens qualifying tournaments, which at this stage we're not quite sure where they're going to be held," he said.
"They just have to play two tournaments I think...they just have to be Olympic qualifying tournaments.
"There's a tournament in the World Cup year [2019] that we still need to find out the minor details of that with Oceania [Rugby], but there's a tournament that we may be able to qualify them in around June/July of World Cup year."
