A winning treble at Rosehill has brought up another milestone for Sydney's leading apprentice Winona Costin.

The 20-year-old outrode her 2kg metropolitan claim with her 50th city win on the John Sargent-trained Queen Of Kariba, the middle leg of her trio.

Her other winners were Intrepid Way for Amanda Turner and Mauro Poletti and Beauty's Beast for Melbourne trainer Danny O'Brien.

Costin began her apprenticeship with Mick Tubman at Kembla Grange and moved to Sydney in 2014, joining the Gai Waterhouse stable.

Earlier this year, she transferred her indentures to Peter and Paul Snowden.

Even before the Rosehill treble, Costin had a stranglehold on the junior premiership going into the meeting with 36 metropolitan wins, 20 more than Brodie Loy.

Her new total of 39 positions her seventh, equal with Tim Clark on the overall tally and seven wins ahead of Kerrin McEvoy whose double in the two staying races at Rosehill takes him to 33.

"I was only just starting to ride some Sydney winners at the start of the season so winning the apprentices premiership wasn't a huge goal for me," Costin said.

"I thought the premiership would be pretty hard to achieve so this has been a season that is over and above my expectations."

Costin showed her versatility to lead all the way on Beauty's Beast while her other two winners came from back in their races.

She will claim 1.5kg from Wednesday's Warwick Farm meeting.