The spokeswoman said there were still available parks at the mall.
"It's just about being a bit patient and being kind," the spokeswoman said.
New Zealand Transport Authority (NZTA) are warning of heavy traffic on the Southern Motorway at the turnoff to Sylvia Park in the Auckland suburb of Mt Wellington.
Another social media user said: "We just arrived by train and looks like a lot of people have opted for this option too so not surprised mall is overflowing. Line to enter car park building is down by Waipuna hotel!"
The Sylvia Park mall has recently opened 600 new parks.
Boxing Day is expected to be one of the biggest shopping days of the year.
More than 280,000 people visited one of Kiwi Property's six shopping centres on Boxing Day last year.
Kiwi Property owns and manages Sylvia Park and LynnMall in Auckland, Centre Place and The Base in Hamilton (jointly with Tainui Group Holdings), The Plaza in Palmerston North and Northlands in Christchurch.