I've done it, but did it as part of a guided group.
We booked through Milford Track Guided Walks and they supplied the packs and rain coats and picked us up and dropped us back to Queenstown.
You will need your own hiking boots (make sure they are well worn in to avoid blisters), thermal underwear, thermal jumper & socks, gloves and maybe hiking poles if you choose.
It rained when we hiked it, so we got soaked through to our skin, it was freezing and there was snow on top of the mountains even though it was only March.
That said, it was a great adventure and my friend and I loved it. Good for first time trekkers too. As we did it through Milford Track Guided Walks, we stayed in a lodge at night and got a cooked dinner, they had drying facilities for your gear, toilet & shower facilities and a nice warm bed to sleep in.
Let me know if you have any other questions, will be happy to give you more info from my experience.