"It took us three months to make them all and the first one, Miss Piggy, went missing four days after they were installed," Mrs Bates said.
"Lego man disappeared a week later and a sky rocket with Rotorua written on the side of it, quite distinctive that one, was taken. You can't help but think, 'What on earth are they are doing with them and why would they want them? They're no use to anybody."
The group wants the covers to be returned to the i-SITE office at City Focus.
"One lady started talking about taking their other [covers] off. Council staff have put cable ties around but they must be cutting the cable ties to get access to them," Mrs Bates said.
"On our Facebook page people have commented on how wonderful it's been with lots of children loving having their photos with them."
Shelly Braddock noticed her double happy fire cracker piece missing last Wednesday.
"I was cheesed off because that was two weeks of work gone. It's ruined the joy for others. People have been amazed by [the yarnbombs], it brings them joy and happiness," she said.
The council's Inner City lead advisor Portia McKenzie said it was disappointing the yarnbombs were damaged.
"We were really disappointed to find that some of the hard work of our creative community to bring colour and vitality into our inner city had been the subject of vandalism," she said.
"However as a result of an examination of CCTV security camera footage, Rotorua police have already apprehended a person and one of the yarn-bomb covers has been recovered to date."
She did not know which cover was recovered.
Police were unavailable for comment yesterday.
