There are only a few operators in Winter. You need these operators to get you from Te Anau Downs to the start of the track. They don’t go if the weather is bad or if the place is frozen up. You might get lucky.
Fiordland Water Taxi: http://www.fiordlandwatertaxi.co.nz/milford-track-transport#contactus
Here is a quote from FirordlandWaterTaxi for the WINTER crossing package:Winter Milford Track Transport
Finally, you should read these articles (below) before attempting the crossing:
See more information at our sister site, http://nzexplorers.com/nz-great-walks/milford-track/