A community member drew my attention to the OpenTopography project and the map that can be created showing erosion and accretion over part of Waikawa Beach between 2013 and 2015.
Topography measures the height of natural features, such as the ground. If you compare how high the ground is between two different years you can detect how much that height has changed.
To generate the map for yourself you will need to watch the very clear and helpful 10-minute explanatory video.
The maps below are what I produced after watching the tutorial.
The first map simply shows the area of interest.
The second map is heights as measured in 2015. This is known as the reference layer. Unfortunately it stops just south of the all-purpose access. The area covered here is a bit smaller than in 2013.
The third map is heights as measured in 2013.
The fourth map compares heights between the two maps. On the beach, red shows erosion, while blue shows accretion. On land the colours indicate height differences too. Notice, for example, the blue cross shape of the building at around about 1 Reay Mackay Grove.
Overall, there is noticeable accretion along the open coast to the south, and clear erosion of the shoreline next to the north end of Reay Mackay Grove and north of that.