In one case, a Wellington Salvation Army church leader, Steve Molen, sent a text to a group about an upcoming dinner saying "bring a date or your muma but would prefer it if you bought a youth and plenty of food should be a great night so see you there six o'clock".
It was mysteriously changed to "bring a date or your muma but would prefer it if you bought me she setting a bad example for me".
Mr Molen contacted the Telecom call centre and was told that he was not the first to come across the problem, The Dominion Post reported.
"[The call centre worker] said it was a software fault that adds lines to the last part of people's texts...and there had even been swear words added on to some messages," Mr Molen said.
"Telecom has a responsibility to make sure that, if you send a text, it is what ends up being sent."
Telecom spokeswoman Rebecca Ingram said the problem was being investigated, but said it was "extremely rare".
Ms Ingram said additional content being added to messages did not indicate cellphones were being hacked.
However, Telecommunications Users Association chief executive Ernie Newman said it seemed likely the phones were being hacked.
"It doesn't seem to me that a random insert into a text message would come out with an intelligible message at the end of it," he said.
