Needn't Have Done (Verb3)
Didn't Need to / Didn't Have to
Didn't Have to / Didn't Need to
If we say that somebody didn’t have to or didn’t need to do something, we mean that the action was not necessary and the person did not do it.
It started raining heavily so I didn’t need to water the flowers. (it wasn’t necessary, so I didn’t do that.)
It was my day off yesterday, that’s why I didn’t need to get up early.(It was not obligatory for me)
We use the base form (verb1) of the verb after the modal "didn't have(need) to".
I didn't need to wake up...
Needn't Have + Verb3
If we say that somebody “needn’t have done” something, we mean that the action is done, but it was unnecessary and even a waste of time.
You needn’t have done the washing up as I was going to do that tomorrow.
When the party was over, I realized that you needn’t have cooked so much food as most of it was untouched.
Yesterday my friend had already bought 2 tickets for the concert so you really needn’t have bought another one, it was a waste of money.
Sandra worked all the weekend to finish her assignment on time, but actually she needn’t have done so.
The bus arrived 2 minutes after you left, so you needn't have taken a cab.
As you can see, in the examples above the actions done were not necessary and actually the efforts were in vain.