Johnston made history for the second time this year after becoming the first player to commit to the Chiefs ahead of their entry to the competition in 2028.
The 31-year-old confirmed the move on Tuesday in an Instagram story post with the words "Yessssss sirrrrrrr".
But the expansion franchise's biggest capture for their inaugural season is landing Luai as their marquee signing, a move the five-eighth is expected to publicly confirm on Wednesday.
The Wests Tigers superstar spent last weekend in PNG and met with Prime Minister James Marape and senior club officials.
"To be the first high-profile player to come and show interest in our team is something we deeply appreciate," Marape told local media.
"It sends a message that Papua New Guinea is ready, rugby league is strong here, and our future in the NRL is bright."
Luai flew to PNG and back on a private jet with his family, touching down in Sydney on Monday evening to a media scrum.
He is due to front a press conference at Wests Tigers headquarters on Wednesday morning where he is expected to confirm he will join the Chiefs in 2028.
A four-time premiership winner with Penrith, the playmaker has the Tigers flying in third spot on the ladder.
But the temptation to be the face of a new franchise and earn a tax-free $1.2 million-a-season deal with the Chiefs appears to have proven too good to turn down.
Luai, 29, will activate an option to spend 2027 with the Tigers for the same money before then heading to Port Moresby and become the face of the Chiefs.
Johnston, meanwhile, will join the new club on a one-year deal understood to be worth a tax-free $325,000.
He passed Ken Irvine's long-standing NRL try-scoring record earlier this season with his 212th and 213th four-pointers for the Rabbitohs.
The 250-game veteran has since moved to a career tally of 219 tries.
Johnston, who will be 33 in 2028, received the blessing of Souths coach Wayne Bennett earlier this year to join the PNG franchise given his strong ties with the country.
The winger made his Test debut for the Kumuls in 2019, scoring four tries in 12 internationals for the league-obsessed nation.
AAP revealed last August that Johnston had the option to sign with the Chiefs before November 1.
In a measure of his standing and popularity at the Rabbitohs, Johnston also has the support of teammates.
"Yeah, he told us this morning. Look, you've got to be happy for the bloke," said prop Sean Keppie.
"He's given so much to the game and to see him get that first position over there is pretty cool to see.
"I'm pretty sure they're going to do some pretty good things over there, so he'll grab it with both hands and he's already scored so many tries, hopefully he can score some more, but not against us."
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