Complainants also claimed it was discriminatory towards, and denigrated, participants and viewers.
The Broadcasting Standards Authority has previously considered complaints from the show, which it found were acceptable for broadcast.
In its latest decision, the authority concluded episodes 3 to 5 of the series, as refered to by the complainants, were not adequately signposted for viewers in the pre-broadcast warning.
It upheld the complaint that a stronger warning was needed "to fairly reflect the nature of the content".
It did not uphold complaints of breaches of good taste and decency.
"In our view, episodes 3 to 5 of this programme contained more explicit discussion of sexual preferences or activities, compared to the first two episodes considered in our previous decision," it said.
"Further, while episodes 3 to 5 could be seen as more lewd or racy, these were broadcast later in the programme's season and following a wide range of media attention and commentary."
It did, however, shut down claims that the episodes were "pornographic".
"We consider that the warning was not sufficient in this case. This programme went beyond nude images and explored sexual issues, albeit in a matter-of-fact way. It is evident from the Adults Only classification and later 9.30pm time of broadcast that the broadcaster identified the stronger content of the programmes.
"Accordingly, notwithstanding our view that overall the content was acceptable for the timeslot, we uphold the Standard 1 complaints on this one ground, that the programmes required a stronger warning to fairly reflect the nature of the content."
In an earlier decision, the Broadcasting Standards Authority concluded the first episodes of the series were acceptable for broadcast, based on the grounds of freedom of expression.
"There was a positive element to this programme, and to say that it should not have been broadcast at all would in our view represent a disproportionate restriction on the right to freedom of expression," the decision said.
"This programme addressed real aspects of human social behaviour. It was descriptive and good humoured in its evaluation of human behaviours and attitudes, and some viewers would have found this programme to be entertaining."
The Broadcasting Standards Authority did accept the show might have not been to everyone's tastes.
"Classifications, the time of broadcast and warnings allow viewers to make an informed choice about what they are exposed to. In this case, while there was a pre-broadcast warning for 'nudity', this was not sufficient to advise viewers about the sexual references made throughout the episodes, which in this case we consider went further than in the first two episodes."
Last year, TVNZ spokeswoman Rachel Howard told the Herald the show rated highly among TVNZ 2's target audience of 18- to 49-year-olds.
Every week an average of 190,000 people in that age bracket watched the show.
"Naked Attraction is screened on TVNZ 2 in a 9.30pm and 9.45pm Adults Only time slot. It contained a clear content and nudity warning before broadcast, giving viewers choice over whether they would like to remain watching the programme or prefer to watch another channel instead," Howard said.
"The programme performed strongly in TVNZ 2's target demographic. However, a decision has not be made on whether future seasons will be screened."
The programme is no longer screened on TV or available on TVNZ On Demand.