For those heading south, the trip between the city and Bombay was taking 51 minutes - 20 minutes longer than usual.
A serious crash took place in Auckland's Waterview Tunnel around 1.30pm. That was followed by another crash on the Southern Motorway at 3pm.
Some northbound lanes near the Waterview Tunnel are blocked as traffic backs up from the tunnel exit back to the Dominion Rd exit.
Ambulance and police teams are at the scene attending to those involved the crash and managing the flow of traffic.
One patient was taken to Auckland City Hospital in a critical condition, a St John spokeswoman said.
The service sent two ambulances and one manager to the crash after being alerted at 1.28pm, she said.
NZTA has asked motorists to avoid the area if possible.
Those who can not are asked to use the right and centre lanes through the tunnel and merge with care.
The crash on the Southern Motorway is blocking the right southbound lane just before the South Eastern Highway off-ramp.
The crashes come as peak traffic is expected to build significantly as some Auckland families look to make an early Friday afternoon getaway for the start of the school holidays.
NZTA warned motorists to expect delays and to allow extra time for journeys.
"Traffic is building around the Auckland Motorway network as school holiday traffic and #FridayTraffic sets in," NZTA tweeted. "Please allow additional time for any travel this PM peak or be prepared for delays."
Police national manager for road policing Gini Welch said the larger volume of traffic on the roads for school holidays increased the chance of serious crashes.
"If you're the driver, put your phone away before you set off, and don't let people in the car distract you from what you're doing. It's your job to get everybody to your destination safely.
"Watch your speed and your following distances ... Make sure you're well rested before you set off.
"Finally, don't be complacent about wearing your seatbelt. Everybody in the car should be wearing a seatbelt or a child safety restraint."
