P1 represents Put through. Search 'Put Through' on this document:
https://www.asx.com.au/documents/rules/asx_or_procedures.pdf
Basically shifting from one nominee account for a large holder to another. Also possibly a broker that has been accumulating on behalf of a client has transferred it to another account of the same client.
My bet it is either OCP or Regal - more likely the former.