Bolton has played just two T20Is in her four-year international career, and her last game was back in September 2016 against Sri Lanka. She was impressive in the last Women's Big Bash League season, where she made 482 runs in 16 games to finish third on the runs table. She was also among the top run-scorers in the Women's Super League in August.
"Nicole Bolton has been a real revelation I think in T20 in the last 12 months," head coach Matthew Mott said. "She was traditionally more of a one-day player but she's reinvented herself, playing some different shots and putting bowlers under pressure. And she's also one of the best fielders in the world at the moment."
Jonassen, who had undergone a minor surgery on her left knee after injuring herself during a warm-up, had missed the three-match T20I series against New Zealand. Mott said her recovery was ahead of schedule, but the selectors have added a "subject to fitness" rider to her inclusion.
"Jess was always going to come back in, being one of the best T20 bowlers in the world," Mott said. "We're still waiting, she's doing some more rehab but it's all been good. She's excited about the progress that's been made and it's ahead of schedule."
Jonasssen and Bolton are the only additions to the squad that completed a T20I whitewash over New Zealand on Friday.
The selectors also named a 14-player squad for the upcoming ODIs against Pakistan. The T20I squad will play three matches against Pakistan in Malaysia after the ODIs, which will serve as a final tune-up ahead of the World T20.
Meg Lanning, the captain, said her squad was in good shape, and the new tactics they had implemented, such as going in with the quicker scorers at the top, had paid off. "We've looked at our strike rates in the last couple of years and really tried to reward players with 130-plus strike-rates and maximise their time in the Powerplay," Lanning said. "We haven't had that success in the last two World Cups [World T20 2016 and Women's World Cup 2017], so we're very determined to make sure we get the result that we want."
Australia finished runners-up in the 2016 World T20, losing to West Indies in the final. Ahead of the tournament, they will play two warm-up matches in the West Indies, against England and South Africa. They are placed in Group B for the World T20, which includes India, Pakistan, New Zealand and Ireland, with all their matches taking place at the Providence Stadium in Guyana.