Isaacs blew through an intersection controlled by a stop sign and hit Ferris' car at an estimated speed of 78kmh, impacting with the right front wheel and driver's door.
The force of the impact caused the victim's vehicle to become airborne and flip through the air.
Sgt Collin said her car took out a stop sign and a fence and ended up on its side against a tree facing the opposite direction.
Ferris was knocked unconscious and was trapped for about an hour before she was freed by emergency services.
It took some time to stabilise her before she was flown to Christchurch Hospital for treatment.
While Isaacs sustained a broken ankle, Ferris sustained serious traumatic injuries, including head injuries, which means she requires assistance to complete daily activities and had not seen her children since the incident.
A blood test revealed Isaacs had both methamphetamine and cannabis in his system - while he admitted having a cannabis cone in Picton, he chose not to comment in relation to the methamphetamine.
He told police had been on his phone at the time of the crash and did not see the stop sign.
Judge McLeod accepted Isaacs' guilty pleas and remanded him on bail ahead of sentencing in March.
Bail conditions include not to possess or consume any controlled drugs, except for those prescribed, and not to drive.
