"The slip could go at any minute. Another big piece could come down,'' he said.
"Bits are coming off it all the time. It's certainly not stable.''
Mr MacLean said residents of eight cordoned homes were allowed just 10 minutes to grab essentials but two homes were considered so unsafe everyone was barred from entering - including experts assessing the slip.
Security guards were being posted outside the cordoned off homes.
Residents of Kilmarnock Heights Rest Home, who were relocated to another facility in Karori, have now been given the all-clear to go back home.
Chris Graham, chief executive of Presbyterian Support Services which runs the rest home, said it had been a long night for the residents.
"Some of them are a bit tired now and want to go home, but they're all okay.
"We're going to give them a lunch and then take them back. Luckily it's a nice day.''
Mr Graham said none of the staff or residents saw the slip occur, but they did detect a strong smell of soil from the eroded bank.
Wellington Mayor Celia Wade-Brown promised every assistance would be given to those families caught up in today's landslide.
"Anyone that needs to be out of a home will be put up," she said.
She was relieved the incident happened without loss of life or injury.
She said GNS scientists, Opus engineers and council staff were working hard to find the cause of the massive landslide.
She praised the swift and co-ordinated response of emergency services to the early morning drama.
Mr MacLean said the council had opened an emergency evacuation centre at the Newtown Community Centre in Colombo St.
Around seven displaced residents were seeking refuge.
Access to the the affected was restricted with council engineers and emergency services assessing the situation.
"It's pretty much a no-go area."
Early reports that the slip was the result of a burst water main were now be reassessed as being a result of the slip rather than the cause.
The slip was also affecting holes on the town belt Berhampore Golf Course.
