Needn’t Have + Past Participle (V3).

Needn’t Have = need not have.

"1. Was not necessary.

You needn't have hurried (= it was not necessary for you to hurry, but you did).

[Need not = needn’t: You needn't finish that work today.]

# AmE: He needn't have gone to hospital, but he went just to reassure himself. (BrE = He did something that wasn't necessary.) 

# He didn't need to go to hospital after all - he only had a few bruises. (BrE = he didn't go.)

2. It was not necessary or useful to do it, although did it.

I was a little nervous when I announced my engagement to Grace, but I needn't have worried.
We spent a hell of a lot of money that we needn't have spent."