Duo to call Melbourne home until 2020
Ainsley joined the Melbourne Rebels after 17 Super Rugby caps for the Western Force, and has played in all nine matches since crossing to Melbourne at the beginning of the 2018 season.
The highly rated 22-year-old featured for the Wallabies in their clash with the Barbarians in Sydney last October but injury cruelled his chances of a Test debut on the 2017 Spring Tour.
Ainsley said he was delighted to commit to his future in Melbourne.
"I'm really excited to be staying here in Melbourne. We've got a really good young group and we’re building something special with Dave [Wessels] as well," Ainsley said.
"I've really enjoyed my time in Melbourne and being part of the world-class environment that the Rebels are building, that's going to make the people of Melbourne proud to support our team."
The 18-year-old outside back, Tupou will also call Melbourne home until 2020 as he looks to add to his three Super Rugby appearances.
Tupou made his top-flight rugby debut last season and was selected in the Junior Wallabies squad for the Oceania Rugby Under-20s Championship tournament. However, injury to senior Rebels players has seen him add to his Super Rugby games tally against the Queensland Reds and Stormers this season.
Tupou said he was excited to continue his career that had given him his top-flight rugby start.
"It's awesome to know I’ll be here at the Rebels for another two years and I thank the club for the faith that they've shown in me," Tupou said.
"It's a really exciting group of players who play with a lot of love for each other, and it's awesome for me to be a part of it.
"I can't wait to keep learning off guys like Reece [Hodge], Will [Genia] and Dane [Haylett-Petty] and taking my opportunities at Super Rugby level."
Melbourne Rebels General Manager – Rugby, Nick Ryan said: "To be able to deliver long term and sustainable success, we need to secure and develop young talent like Jermaine and Semisi.
"We are excited by their performances to date, potential and their commitment to the community and look forward to developing them over the next few seasons."