Is a pro rata payment a one-off payment?

No. Quite often part time job salaries are advertised as if the job was full-time. Easiest to explain this with an example.

A job might be advertised as 20000 pro rata. If the job was 3 days a week your actual gross salary would be 12000 (20 / 5 x 3). If the job was advertised as 25k pro rata and you worked 3 days your actual gross salary would be 15000.