Sent in to bat first, SA struggled on a lively pitch and was dismissed for 152 off 44 overs. Thomas Kelly was the top-scorer with 35.
NSW Metro was 3/48 but steadied before two quick wickets gave SA some hope with the score at 5/112. Another partnership, however, steered NSW Metro to the win and it reached the target in the 30th over, six wickets down.
In the other under-17s semi-final, Queensland bowled the Cricket Australia XI out for 87 and then recovered from a shaky start to pass the total with six wickets in hand.
Queensland and NSW Metro will meet in the final on Thursday.
For fixtures and results, visit the National Championships website.