Plant fossil in rock

Plant fossil in rock from Motunau beach. Another image below.

Can anyone help me identify this plant fossil visible on the surface of this rock?  Research has not helped. I am just curious as to what the plant may be and which geological era it comes from. It would be great to find out!

The rock has been in my studio and unfortunately was splashed with ink and paint, so ignore the grey and yellow marks – the fossil appears in the pale grey markings. One of the leaves (?) bottom left is quite visible.

I brought the rock home as I was interested in the sea creature holes on the other side, and only later noticed the markings when I tried to remove the grey circles and the yellow stain.

The rock comes from Motunau beach north of Christchurch, and I attach some information from a really useful book for the uninitiated about the geology of the area which comes from the “Field Guide to NZ Geology” by Jocelyn Thornton.

Another image of the rock, adjusted to increase visibility of the fossil