I think that locally the drop down from SH 1 just north of Hunterville is called Vinegar Hill. This is a great diversion if riding down SH 1 from north of Hunterville to Fielding and Palmerston North. I must admit I´ve never ridden it from south to north, but it should be equally enjoyable.
Did this run two-up yesterday on our way from the Wairarapa to Taupo. Excellent fun. (Triumph 1050 Tiger)
Make sure to stop at the Stormy Point Lookout for fantastic views of the Rangateki Valley