Authorities came to the conclusion after comparing crime statistics between 2013 and 2016.
Police have said that “on occasion crimes have been registered in which the perpetrators have have acted from within larger groups,” Die Welt reported on Friday
These type of crime is “notable” because sexual crimes are usually "almost exclusively” committed by people acting alone.
In all the instances of crimes carried out by groups - what the police describe as "a new form of criminality" - the perpetrators were not German, police say.
The statistics also show that in the second quarter of 2016, no more group sexual assaults at swimming pools were registered in Lower Saxony.
Police believe this is due to members of the public being more prepared to report sexual assault in the wake of mass assaults over New Year in Cologne and swifter reaction from police officers.
Whereas around 50 sexual crimes were recorded at swimming pools in the northern state in the previous year, after six months of 2016 “the number is already more than half the average for previous years".
So far in 2016 “a majority of suspects were non-German” whereas in previous years “a clear majority were German", police said.
103 sex crime at pools in NRW
Speaking to the Rheinische Post (RP), the state office for Central Police Services (LZPD) said that a total of 103 sex crimes have been reported at pools across North Rhine-Westphalia since the start of 2016.
"The allegations range from insults to exhibitionism to assault and rape," an LZPD spokesperson told the paper. The speaker added that a "disproportionate" number of the allegations were made against men with a migrant background.
In 2014, there were seven cases of sex crimes reported at swimming pools in Düsseldorf, with an increase to 17 in 2015. In 2016 so far, police in the state capital have received eight allegations of sex crimes happening in swimming pools.
In an internal email seen by Bild, police described the increase as "enormous."
One 20-year-old Syrian man is facing court because he grabbed and kissed the arm of a 14-year-old. The RP reported that offences committed by Germans include touching female swimmers underwater as well as exhibitionism with public masturbation.
A spokesperson for the association of swimming pools in Düsseldorf told RP she knew of no cases of sexual assault or rape which had taken place at their pools.
In Cologne, where hundreds of women reported being sexually assaulted on New Year's Eve at the main train station, the central swimming pool authority said they hadn't noticed an increase. However, spokesperson Achim Fischer conceded that "there are of course situations that are not reported to us, but go directly to the police".
Green party refugee politics spokesperson Monika Düker said that the report was not a reason to ban migrants from swimming pools.
"Any sexual assault in our swimming pools is one too many. However, to look at the present numbers and speak of an 'enormous increase' or even a 'sex mob' is an unjustified overstatement," she said.
In January, one NRW swimming pool in Bornheim banned male refugees because some swimmers felt threatened by their presence. Social services then stepped in with pool etiquette courses, and the ban was lifted after five days.
"Our measures worked," said a city spokesperson.