The plant shown in the attached photo is growing in muddy sand on the Waikanae River estuary. It is low growing, has yellow flowers in summer, gets periodically flooded by either a very high tide or a river flood. There are no other plants growing nearby. I wondered if it is a Cotula or Leptinella.
The photo is of a plant growing in my garden [100% sand]. This plant is a little more vigorous than the plants growing on the river.