A police spokesperson said the earlier crash happened as the vehicle was heading north on the motorway.
"Two occupants of the vehicle were taken to hospital following the crash. The driver has since been assessed as being in a critical condition."
The lane closure caused long delays for drivers between Drury and Papatoetoe.
NZTA Waka Kotahi asked commuters to seek an alternative route to work to avoid congestion.
The SH20 crash happened at 6.30am and partially blocked lanes.
The traffic jam appears to be around the Dominion Rd off-ramp but could have since worsened.
"Motorists should expect delays and we appreciate their cooperation as scenes are cleared," police said.