Well without evidence (as you will know of course) its hard to make a call on your description. Carex buchananii - least ways in the North Island is pretty scarce, and it likes fresh water systems, on lake, tarn and river margins, in well lighted places but also on the margin of riparian forest. In habit the plants are pretty stout, the culms erect, and the leaves usually wine-red or red-grey, the fruiting culms are erect - never drooping, and often partially obscured within the erect leaves. What were the leaves like in your plant - what colour? Were the channeled, folded or double-folded - those sorts of observations will help. Also be aware that a common sedge in those areas is an unnamed entity oft known as Carex "raotest" - its pretty robust but the culms (like C. testacea, C. devia, C. flagellifera and C. raoulii) elongated greatly, trailing on the ground as 'trip-me-ups' (a sensible vernacular for once).