The Australian Competition and Consumer Commission (ACCC) was expected to give the venture the go-ahead in February but then pushed the date to April.
The latest delay is because the ACCC wants to allow further time for miners to respond to an information request issued in March.
The competition watchdog will make an announcement on 27 May.
The investigation centres around Western Australian mining operations held by both BHP and Rio Tinto.
The European Commission and the Japan fair trade commission have both expressed concerns over the merger, arguing that it would give Rio Tinto and BHP too much say over iron ore pricing.