Te Waikoropupū Springs in Golden Bay is one of the freshest springs in New Zealand and the world. To get there from Tākaka, take Pupu Valley Road, then Pupu Springs Road to the short loop track (an hour at the most) around the springs and the stream that feeds it.
At the start of the trail, an excellent series of panels describes the springs, their effective destruction by gold mining in colonial times(!), and their subsequent restoration. When you look into the pools, with their amazing depth of view, it is hard to imagine they have not always been this pristine. The pools are also wāhi tapu for local Māori and are guarded by a taniwha, Huriawa. Don't even think about swimming!
This is a very popular walk, so try to avoid peak times.
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