Why does your tide forecaster appear to provide times that are 20 – 60 minutes different to most other sources?

The example used here is for Murawai Beach, 11km north of Anawhata.

This is a common question. The Metservice tide forecast is based on LINZ tide forecast, which uses known tide times at standard ports and then applies an offset for 'secondary ports' - for example, Muriwai Beach. These offset values are often approximate.

The NIWA tide forecast is predicted by a purpose built tide model designed to calculate the tide at any location of open coast (not harbours) around New Zealand. The NIWA tide model has a stated accuracy of +/- 10 mins.

The best way to check the accuracy of predicted tides is to compare them with actual measured data. NIWA operates an open coast tide gauge at Anawhata which is approx. 11 km south of Muriwai Beach.

Example: on the 5th of August, low tide occurred at 17:45 at Anawhata. If one assumes a small offset of ~5 mins for the 11 km difference and takes into account that the tide propagates around NZ in an anticlockwise rotation, then the predicted tide at Murawai Beach (based on the Anawhata sea-level gauge) would be 17:45 – 5 mins = 17:40.

This is very close to the NIWA prediction (17:41) for the day.