The Newcastle Knights NRLW star told her close family and friends, but had to keep her planned selection largely under wraps.

However for the 18-year-old playmaker, whose sister Hannah played in the first rebranded women's State of Origin in 2018, getting an early heads up about her selection made officially finding out she had made the side no less exciting.

"I found out a couple of months ago, but it wasn't for sure, for sure," Southwell told the Newcastle Herald, following her first training session with the Sky Blues in Sydney on Thursday.

"Anything can happen between then and when the selection comes out.

"[NSW Sky Blues coach] Kylie [Hilder] messaged me and said she wanted to catch up ... and she told me I'd be playing in Origin.

"I was stoked.

"I couldn't stop saying thank you, couldn't keep the smile off my face for about four hours afterwards.

"Anything can happen between then and when the actual selections come out, so I didn't want to get my hopes up too much.

"But it was so cool. To see the NSW Blues [social media] post with a photo of [Sky Blues captain] Isabelle [Kelly] and my name on that list, was absolutely amazing."

Southwell said it was "bittersweet" telling her older sister she was going to make the side, given Hannah is unavailable due to injury.

"That's all I really wanted to do, was play with my sister," she said. "That's a very big goal for me in the future."

Southwell has only played with two NSW players before, fellow Knight Yasmin Clydsdale and former clubmate Millie Boyle, so getting to know her teammates will be a priority this week.

"Just building combos and getting reps in playing alongside them," she said.

The Cameron Park product will play five-eighth against Queensland at CommBank Stadium on Thursday.

She said the position would be much the same as her role at halfback for the Knights, but admitted to a few nerves ahead of playing in the high-intensity fixture.

"It's the pinnacle of our sport," she said. "I'm excited. A bit nervous but definitely excited to play some footy. That's all I really want to do. We've had a stint not playing footy so I just want to play and have some fun."

Meanwhile, the Knights on Thursday announced five more players for their upcoming NRLW campaign.

NSW Country prop Viena Tinao has been included in the top 24-player roster, while Tylah Vallance, Jacinta Carter, Tazmyne Luschwitz and Evie Jones - all existing lower-grade players - take the club's four NRLW development deals this season.

Tinao, 21, played for Illawarra in the recently concluded state competition.

"Her game evolved weekly and her commitment to nailing those all-important effort areas became increasingly apparent as the season went on," Knights coach Ronald Griffiths said. "She is a quiet, hardworking and humble person who we see as a great fit for our team and our club."

The NRLW squad begin pre-season training next week. Newcastle begin their campaign against the Dragons at home on July 22.