The best time to walk there in my opinion is in afternoon as at that time sun very nice lightens Southern Alps.
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So if you are going Milford Sound cruise in the morning, on your way back stop in The Divide car park where trail starts. There also begins Routeburn Track, but after you pass beech forest Routeburn and Key Summit tracks will split. Track is well signed and there is no way to get lost there. It takes about 40-60 minutes one way to get to summit and I would grade it as easy walk comparing with Lake Marian track
Some more photos from Fiordland in New Zealand