Interesting to see the threshers about down there. Threshers and broadbill are often found in the same habitats so maybe mulletman could be right. Theres a backpackers up on the peninsula rd (barnacle bills??) that has a large bill on the mantle piece that purportedly came from a broadie out on the Kaikoura trench some years back.
15lb is not a big schoolie, I caught one back in november down by the gravel pits in kaikoura that would have gone 50lb easy. Bait was a large strip of kahawai.