The Dubbo Roos under 15s Blue and under 17s Blue sides will both play in this season's grand finals after both crushed Mudgee in their respective major semis at No. 1 Oval.

The under 15s continued their stellar unbeaten campaign with a simply dominant 70-12 victory while the 17s, playing against a Wombats side which claimed this year's minor premiership, scored a 38-12 win.

"The boys have had a remarkable year," under 15s coach James Austin said.

"They haven't lost a game, they've only conceded about 11 tries, and they've scored a lot.

"But that means nothing now unless you win on grand final day."

Austin and his side went into the match with plenty of confidence after such a commanding run this season, but even he wasn't expecting the kind of result served up.

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As has been the case so many times this year, every player on the field delivered for the Roos as they raced to a strong 35-5 half-time lead before running away with the game in the second stanza.

"We've been sitting down and trying to figure out the end of season awards and it's so hard because of that team aspect," Austin said.

"They just keep going and set individual goals and team goals. Most of them have been together since they were seven so they've played a lot of footy together."

The players featured in the lower age Reds team last year and only just missed out on a grand final then, but this year things have gone to a new level.

The biggest battle Austin faces in the lead-up to the grand final is a mounting injury list, with two more players going down in last weekend's win.

"I've got a lock we lost three weeks ago with ligament damage in his ankle, a prop with a broken toe, another prop with a dislocated shoulder, a hooker with an injured shoulder, a lock with a scapula injury," he said.

"It's about managing those but I'm confident we've got enough depth to get the job done."

The 17s Blues side has also dealt with a stack of injuries this season.

That was partly the reason they missed missed out on top spot on the ladder but the Roos had players back on deck on Saturday and it showed against the minor premiers.

"I was confident the boys could get the win but I thought it would be close," Austin said of the successful Jason Blake-coached 17s.

"But it also wasn't a surprise to see them win by that much, if that makes sense. Jason has had to deal with a lot of injuries and representative commitments.

"They've done well to make the grand final and if they can get the best 15 out there then they're unstoppable."

Both Dubbo sides find out their grand final opponents this weekend, with preliminary finals being played around Saturday's National Rugby Championship fixture at Apex Oval.