a quick look at the german penal code would suggest that - at least if the model is female - there doesn't appear to be a law that would prohibit this as long as no sexual act is involved. 
Don't considre this legal advice - just a quick non-professional reaction to your question. It may well be that there are local administrative rules prohibiting this and providing the basis for monetary penalties though.

In Austria you can do anything.
Except getting caught. ;)

https://anwaltauskunft.de/magazin/leben/freizeit-alltag/554/freikoerperkultur-wann-man-wo-nackt-sein-darf/

This article claims that while you may not do it, you only have to pay a fine, and only if someone calls the police. And it is a very vague law, so it depends on the judge.

http://de.wikipedia.org/wiki/Oben_ohne

And the German Topless Article mentions again, it is only an offence if someone is offended, and it is allowed near the beach. And mostly, there is nobody to feel offended by a topless person.

@ Wilko
since the 1950`s
Men = exhibitionism
Women= it´s not

sounds funny but its real :-)

lots of photographers in the german gouverment in the 50th  :-)

I got caught once... got a notification from the court and had to pay about 85€. My delict was called "Störung des öffentlichen Friedens" (Disturbance of the public peace)

Honestly ... a public male nude appears as an exhibitionst very easily.
But i found something working .... as a sculpture, or nearby skulpures or other art works interacting with them.

Sad that this is impossible :s
(or doing it at 3:00 in the morning :p)

So I am quite curious how the law in German is regarding such situations? In all the NIP shootings model and photographer were just lucky there was no police near, or is it legal in Germany to walk the street without clothes... ?

It depends.

Performing sexual acts (sexual intercourse and suchlike) in public is prohibited by criminal law (§183a StGB) IF this act deliberately or knowingly causes public nuisance. (This is a necessary qualification to accomplish this offense!) Punishment is a sentence up to one year in prison or fine.

This offense is prosecuted rather seldom in Germany*. Usually police simply will request people to stop what they are doing.

Now this given above is rather irrelevant for "nude in public"-photographers and models.

There is another law that makes it a petty offense ("Ordnungswidrigkeit") if someone is "harassing the public" - or in german legal poetry: "Belästigung der Allgemeinheit". (§118 OwiG) This can be punished with a penalty charge up to 50,000 Euros.

This law makes it illegal to undertake "rudely indecent actions" in public IF these acts "are fit to harass the public" AND to endanger public order with those actions.

This offense is prosecuted rather seldom, too.**

So if you are shooting NIP at Sunday morning in front of a church in a Bavarian small town chances are bigger to get intro trouble than if you shooting NIP somewhere in Berlin or Hamburg. Chances are rather small that someone will call the police in a big city when somebody is shooting NIP. And if the police should intervene most probable they will request you to stop shooting and leave the place.

Kink.com by the way shot some videos for their "Public Disgrace" site in Berlin and Munich. Without getting trouble.

_________________________________________

*)  The key factor is that prosecuters are obliged to prove that this action had been undertaken with the purpose of causing public nuisance or that causing public nuisance had been approved eventually. Most often this is impossible.
**) The key factor is that prosecuters are obliged to prove that public order had been affected by those actions. That might be the case if a NIP shooting caused a big crowd of people which caused a significant traffic obstruction or so.

since the 1950`s
Men = exhibitionism
Women= it´s not

sounds funny but its real :-)

This applies only to "exhibitionistic acts". (§183 StGB = showing penises in public to other people)

§183a StGB - Performing sexual acts (sexual intercourse and suchlike) in public - applies to men and women equally.

http://www.krone.at/welt/aegypten-nacktfoto-kostet-model-fast-die-freiheit-festnahme-in-tempel-story-588498
Das Löschen der Fotos durch den Fotografen rettete die beiden.