Three former Wallabies and five All Blacks make Tonga's squad its best ever and will give the Pacific nation a "massive X-factor" at the Rugby World Cup, coach Toutai Kefu has said.

Kefu has made the most of World Rugby rules that allow a capped player to switch allegiance to another eligible country following a three-year cooling-off period.

It has seen ex-Wallabies Israel Folau, Adam Coleman and Lopeti Timani make the move to Tonga.

They have been joined by former All Blacks Charles Piutau, Vaea Fifita, Malakai Fekitoa, George Moala and Augustine Pulu.

All were included in Tonga's squad named Wednesday for July Tests against Fiji and Japan in the build-up to the World Cup in France in September.

"This is obviously a different class of side compared to teams we have had in the past and what it enables us to do, it provides us with some massive X-factor," Kefu, a former Wallabies No.8, told The Sydney Morning Herald.

"It was pretty easy, to tell you the truth -- they were all pretty keen to play for Tonga.

"Most of these boys, like Piutau and Fekitoa, they all grew up in Tonga until the age of 14 and 15 and then they moved overseas, so they were all really keen."

Coleman -- whose late father Pau'u Afeaki was Tonga skipper -- played 38 times for Australia and will bring plenty of experience.

Coleman captained English Premiership side London Irish last season before the club recently filed for administration.

"He had some issues obviously with his club side folding recently, so that's been a bit of a distraction," said Kefu.

"But most of the conversations we have had have revolved around him being really passionate to represent that side of his family."

Tonga are in a tough World Cup Group B that also includes Ireland, defending champions South Africa, Scotland and Romania. But Kefu believes they can cause some upsets.

"It certainly can be done," he said.

"I am a person that believes the world is your oyster. I believe in thinking positively, and we have the cattle now.

"And with the level of player comes a higher level of expectation."

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