The Greek word John uses here is not to be confused with Jesus’ word on the cross in John 19:30. Whereas Jesus had spoken about completion (of his mandate), the Lord God here declares (literally), It has happened.
We wonder, What has happened? The term John uses in the present passage is the same word the angel used in Revelation 4:1 when John was instructed to enter heaven: “I will show you what must take place after this.” In the present passage the Lord declares that it has taken place! All the events described in the chapters following Revelation 4 receive their climax and culmination in the declaration of Revelation 21:5: “Behold, I am making all things new!” With all the other things done, this may be considered done also. The following words confirm that line of thought.
6 And he said to me, “It is done! I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give from the spring of the water of life without payment.