The Royals beat Cashmere Technical 3-1 at the Logan Park Turf to ensure they will play in the final of the inaugural competition.
It was a rematch of the thrilling Kate Sheppard Cup quarterfinal of two weeks ago when the Dunedin team won in extra time.
As they did that day, Dias and Shontelle Smith scored goals for the Royals yesterday.
Smith fired in a free kick after just 11 minutes, but Cashmere responded quickly, and it was 1-1 at halftime.
The second half threatened to become a stalemate until Dias popped up to fire in two late goals.
"We made a bit of a tactical change there," Royals coach Graeme Smaill said.
"We moved Margi from the wing into central striker, and it paid dividends."
The Royals have been utterly dominant in local football, scoring 86 goals and conceding just one.
They had to work exceptionally hard to see off Cashmere’s challenge, and that was the point of the new South Island championship, Smaill said.
"This has been a long time coming.
"It’s positive to get this level of competition where the players have to really push themselves."
In the other game in Dunedin yesterday, Roslyn-Wakari was no match for Coast Spirit as the Christchurch club ran away with a 7-0 win.
A double-header in Christchurch this weekend will feature the Royals playing Coastal, and Roslyn-Wakari playing Cashmere.
The Royals-Coastal game is effectively a preview of the South Island final.
A tough period for the Royals will then feature the Kate Sheppard Cup semifinal against Northern Rovers in Auckland on August 20.
