Thomas takes on the role of defence and skills coach as part of the Rugby World Cup Coaching Internship Programme, which is supported part of the International Olympic Committee's international development grant to World Rugby.
The former Wallaroo playmaker will join newly-appointed head coach Senirusi Seruvakula and the wider management team as the Fijiana prepare to compete in their first-ever Rugby World Cup.
"I am extremely excited to be appointed as an assistant coach of the Fijiana 15s team and take part in the World Rugby Coaching Internship Programme, Thomas said.
"Fiji Rugby has been creating a great pathway for women and girls and to be a part of their first Rugby World Cup is going to be a great experience.
"I'm looking forward to working with Seni, he is an experienced coach having coached the Fijian Drua within Australia's National Rugby Championship, winning the premiership in 2018, as well as working with the Flying Fijians."
The Internship Programme was established following a comprehensive review, which highlighted a significant lack of female representation in high-performance coaching across both the men's and women's game.
As part of its strategic plan for the women's game, World Rugby wants at least 40 per cent of all coaches at Rugby World Cup 2025 to be women.
"It is fantastic to see both the Fiji Rugby Union and Rugby Australia working in collaboration in providing Fiji with a suitable elite female candidate to take up the Rugby World Cup 2021 coach internship position with Fijiana 15s," said World Rugby General Manager for Women's Rugby Katie Sadleir.
Thomas was named Australia's Community Coach of the Year in 2017 and has coached the Melbourne Rebels Super W side since 2018.
The Fiji Rugby Union's General Manager of High Performance, Simon Raiwalui, said it was excited to welcome her into the Fijiana fold.
"Our newly-appointed head coach Senirusi Seruvakula has assembled an exciting diverse young coaching staff and we are very fortunate to be able to have someone with the skills and experience such as Alana," he said.
"One of the main priorities within the Fiji Rugby Union is the development of women's rugby within the country and Alana's involvement will assist in the growth of our pathways for our women and girls.
Six other countries have already confirmed their Rugby World Cup coaching interns for next year, with tournament hosts New Zealand and Wales to follow suit in the near future.
Three qualifying spots remain up for grabs, with the tournament scheduled to kick-off in July.