Needn't have and didn't need to

Both are used for about past events, but have a little diference. When used "needn`t", say that something "needn`t done", it`s means that they, but isn`t necessary.
For example:
. You needn't have washed the dishes. I would've put them in the dishwasher.
. I needn't have prepared all that food. They phoned to say they wouldn't be coming.

When "didn`t need to", sometimes is used in the sama meaning. For example:
. You didn't need to wash the dishes. I would've put them in the dishwasher.
. I didn't need to prepare all that food. They phoned to say they wouldn't be coming.

But other times, "didn`t need to" is used to say that something was not necessary to do based on the circumstances where it was. For example:
. The sun came out so we didn't need to take any rainwear on the trip.
. We had plenty of petrol in the tank so I didn't need to fill up.

Needn't and don't need to

Both is used to describe present situations. We can used to give permission to someone not to do something in immediate future. For example:
. You don't need to water the garden this evening. It's going to rain tonight.
. You needn't water the garden this evening. It's going to rain tonight.

But when are talking about general necessity, normaly used "don`t need to"
. You don't need to pay for medical care in National Health Service hospitals.

Need

Need can used like a modal verb or as ordinry verb. When "need" is a modal auxiliary verb it is always used in negative senteces. For example:
. Need I say more? I would like you to stay.

When it is used as an ordinary verb with to before the following infinitive and with an s in the third person singular
.She's almost dehydrated. She needs a drink.