Even though it's more popular in Australia than in other countries, the idiom is still pretty interesting:
— adjective (sometimes hyphenated): very busy; preoccupied with many different things at once.
«Sorry I couldn't make it to lunch today, I've been flat chat with the kids all day.»
— adverb (sometimes hyphenated): at the greatest possible speed, efficiency, or power.
«The whole team is working flat-chat to get this project finished and perfect.»