Johnston has 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 main priority is landing Luai as their inaugural marquee signing.
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 is expected to front a press conference at Wests Tigers headquarters on Wednesday when he is set to reveal his future beyond the 2027 season.
A four-time premiership winner with Penrith, the playmaker has the Tigers flying in third spot on the ladder.
While the 29-year-old would undoubtedly serve as a perfect talismanic first leader for the Chiefs, any defection from the former NSW State of Origin five-eighth would be a shattering blow for success-starved Tigers fans.
Luai must must decide in the next 48 hours whether to accept a lucrative - and tax-free - $1.2 million-a-season deal with the Chiefs.
He has an option for 2027 with the Tigers for the same money but needs to enact it by Thursday.
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.
"I'm really happy for him," Bennett told Nine Media on Tuesday.
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."
14°C