According to the network, Tracy Vo is set to join the team as newsreader, Tim Davies as weather presenter, Alex Cullen as sports presenter and Brooke Boney as entertainment presenter.
Sylvia Jeffreys will permanently replace Sonia Kruger on Today Extra, after the 54-year-old TV veteran resigned following eight years on the show to return to Channel Seven.
Sylvia will host Today Extra alongside David Campbell, and will be covered by journalist Belinda Russell while she is on maternity leave next year.
Weekend Today will be hosted by Richard Wilkins and Rebecca Maddern, with Lara Vella covering Jayne Azzopardi as newsreader while she's also on maternity leave.
Completing the Weekend Today line-up is former Kids' WB presenter Lauren Phillips.
Channel Nine is banking on Karl's return to the morning TV show to boost viewer numbers, after they dipped when he was ousted earlier this year.
Nine's Director of Morning Television, Steven Burling said of the new line-up on Wednesday: "Ally and Karl are quite simply two of the best news broadcasters in the country. Both have reported from conflict zones, they are more than comfortable going toe-to-toe against our most formidable politicians and quite honestly they're who you'd turn to in times of breaking news, be it here in Australia or anywhere in the world," he said.
"Importantly though, Ally and Karl also know how to have a good laugh, they both have an infectious, self-deprecating sense of humour that I believe will resonate with viewers and the rest of the on-air team.
"We've brought together a team that Australia will love waking up with in 2020. As one of Perth's most recognisable news presenters, Tracy will anchor the news bulletins; Tim has resonated with viewers presenting weather on weekends, balancing light hearted segments with the sometimes dire forecasts covering bushfires and weather disasters. Alex Cullen is a trusted voice on Australian television and we welcome him to Today, where his recent fill-in stints have won over viewers. And Brooke will continue the great work of delivering entertainment news and interviewing some of the world's biggest stars."