By looking at penal law, we see, that simple nudity is not a criminal offence: In penal law, all offences, that may be related to nudity, concern only sexual deeds in public. In Austria, penal law is therefore not applicable to mere nudity like hikes in the buff etc.

The Austrian Criminal Code contains the prohibition of a sexual deed in public:

Section 218.
Sexual disturbance and sexual deeds in public

(1) Anyone, who disturbes a person by a means of a sexual deed
1. at her / him or
2. in front of her / him
in cases, where it is applicable to cause a justified public nuisance, shall be punished by imprisonment of up to six months or by a fine of up to 360 day rates, if the offence is not more severely punishable under any other provision.

(1a) According to para. 1, someone is also to be punished, who violates another person's dignity through intensive contact with regard to her / his sexuality.

(2) Anyone is also to be punished, who commits a sexual deed in public and under circumstances, in which her / his behavior is likely to provoke justified nuisance due to direct perception.

(2a) Anyone, who knowingly participates in a gathering of several people, that seeks to commit a sexual disturbance according to para. 1 no. 1 or para. 1a, is, if such a deed has been committed, to be imprisoned for up to one year or to be punished with a fine of up to 720 daily rates.

(2b) Anyone, who commits a sexual disturbance pursuant to para. 1 no. 1 or para. 1a together with at least one other person collusively, shall be punished with imprisonment of up to two years.

(3) In the case of para. 1 and 1a, the contravener is to be prosecuted only with the authorisation of the injured person.

Source: “Section 218 StGB ‘Sexuelle Belästigung und öffentliche geschlechtliche Handlungen’, StGB – Strafgesetzbuch” [en: “Section 218 StGB ‘Sexual harassment and public sexual deeds’, penal code”] (jusline.at)

Penal law is in Austria not applicable to activities such as hiking in the buff.