She said she saw growing numbers sleeping rough even in Parnell, one of Auckland's wealthiest areas.
"We experience it more and more here with people sleeping around the cathedral precinct," she said. "It's not uncommon here to be offering a bowl of cereal in the morning or a warm cup of tea."
She said rough sleepers had increased in the six years she has been there.
"When I first came here it was an irregular thing, now I would describe it as regular," she said.
"It's mostly single people but we've had families sleeping in cars. That would be about a year ago when I personally directly encountered it."
Johnston said the event was initiated by Parnell residents and was not aimed solely at John Key.
"I think it's quite significant that this is one of the wealthiest suburbs in Auckland. John Key has got quite a big house and is very comfortable, yet in his own city people are still living in cars," she said.
"I think it's important that we do do something in Parnell. We will certainly be inviting him again."
The Parnell event is the eighth park-up announced so far, with others also being planned in Glen Innes and possibly elsewhere. The first seven were in Mangere, Otara, Wellington, Onehunga, West Auckland, Hamilton and Napier.
MPs have been invited to all of them but Johnston said no National MPs had turned up so far.