The slip has been growing intensely over the past month after construction crews started work on the area.
Heavy rain in recent months has been blamed for causing part of the carpark to slip away, but Auckland Transport says there's no immediate danger to the public.
Birkenhead Brewing Company spokesman Steve Simms says it's a big issue.
"We've been watching it develop and every car park that disappears is an economic benefit that disappears with it.
"We wake up this morning and the whole thing's about to go. We were standing there this afternoon and we were hearing trees popping inside the forest, so the thing's on the move.
"The next thing that we're gonna see is that it's gonna be a quite spectacular fall, when we see this thing go," Simms said.
Auckland Transport spokesman Mark Hannan says businesses and residents should not worry about the landslip.
"It's on fill, so we've cordoned off the area that's fill, its really old, probably dates back to the Birkenhead council, 70s probably. So it's the fill that has given way.
"We've moved the fence back and we have security guards on and have closed a larger part of the car park," Hannan said.
Simms is disappointed with the "lack of leadership" the council has shown.
"They [the construction team] came across an underwater lake or something while they were driving the piles in around four weeks ago, that's when the cracks started to appear worse. It's just gone 'boom'.
"Our business association ... there has been harping on for the council to take Birkenhead seriously and instead they've been diverted to other areas of Auckland that seem a little more interesting.
"It's starting to scare business away from coming to Birkenhead, and it's scaring customers away from Birkenhead," he said.
Auckland Transport is unsure how the situation will unfold, but it has experts on site assessing the matter.
In a statement, Auckland Transport said parking had been restricted at the carpark.
"Part of the carpark has slipped away after heavy rain in recent months," the statement said.
AT is "actively monitoring" the slip, and an engineering geologist has been on site, it said.
"Engineers are working to determine the stability of the fill that the car park has been built on.
"There is no immediate danger, but a large area of the car park has been cordoned off. Members of the public must not enter the cordoned off area."
