The son of Reliable Man was having his fourth raceday start, having placed in two of his previous outings.
The gelding was positive out of the gates for jockey Ashvin Goindasamy, settling just behind the speed.
Goindasamy navigated his charge five-wide turning for home where he was able to comfortably reel in his rivals and run away to win by a long neck over Principessa, with a further length back to Divine Sava in third.
Trainers Simon and Katrina Alexander were pleased with Rosberg 's winning effort, showing his versatility.
"It was very pleasing to see," Katrina Alexander said. "I have always been a little bit concerned with him on top of the ground, but he has shown he is comfortable with that, having earlier shown that he also cops wet tracks.
"He is an incredibly honest little horse. He is not a big horse, but he has got a big heart and he really is still learning what it is all about.
"He is a really nice horse to train and is 100 percent genuine, which is really pleasing."
While no solid plans have been made after Wednesday's victory, Alexander hasn't ruled out a path towards the Gr.1 New Zealand Derby (2400m) at Ellerslie next March despite the Gerry Harvey bred and owned gelding missing from the initial nominations.
"Distance will be his best friend in the future," Alexander said.
"Ashvin said that after the line he just wanted to go again and he pulled up down the back straight.
"Just on his breeding, being by Reliable Man, they are comfortable at a mile to 2000m at least."We haven't made too many plans past here, we just wanted to tick this box and to see him doing it over 1400m. Knowing it is a bit short for him gives us a bit to work with."