Former back-rower Mellissa Berry has been shifted to the backs where she will partner Clare Flowers in the centres while Elen Evans returns to international duty on the wing.
But after hours of deliberation, Lewis believes he has got the right balance in his team to take on Australia.
He said: "You've got 26 players here who are all desperate to be on the field and who are all desperate for a place to play.
"We're very lucky that with a squad of 26 every player could genuinely be in the starting line-up.
"It's been a tough decision naming the side and it's not been taken lightly. There was debate over a lot of positions but we're confident we have selected a side which will be competitive against Australia.
"We've had a number of warm-up games leading into the World Cup campaign and Sioned has shown herself to be a fantastic, tenacious back row player. She is very much in the mould of Jamie Kift and she's a real asset to the team.
"Dominating the contact area is hugely important and those sides that deal with the contact area will have a distinct advantage and in Jamie, Catrina and Sioned we have three players who are all fantastic in that area.
Wales Women - v Australia
15 Non Evans (Cardiff Quins)
14 Elen Evans (Dolgellau)
13 Mellissa Berry (Blaydon - Co-capt)
12 Clare Flowers (Bristol)
11 Mared Evans (UWIC)
10 Naomi Thomas (Bristol)
9 Laura Prosser (Pontyclun)
8 Jamie Kift (Bristol - Co-capt)
7 Sioned Harries (UWIC)
6 Catrina Nicholas (Cardiff Quins)
5 Rachel Taylor (Cardiff Quins)
4 Gemma Hallett (Pontyclun)
3 Catrin Edwards (Cardiff Quins)
2 Rhian Bowden (UWIC)
1 Jennifer Davies (Waterloo).
Replacements: Caryl Thomas (Bath), Lowri Harries (UWIC), Shona Powell Hughes (Neath Athletic), Lisa Newton (UWIC), Amy Day (UWIC), Awen Thomas (Unattached), Ceri Redman (UWIC)