It's been nine days since Paula McGregor's son Denver Chance was last seen or heard from.
Senior Sergeant Megan Goldie said in a statement on Tuesday that time is running out for Mr Chance.
"Police are still treating this as a missing person's case, but as time passes us by, we are becoming more concerned that something may have happened to him, "
Ms McGregor told Radio New Zealand on Tuesday that she's living in a "nightmare".
"It's terribly out of character for him, we're in touch all the time, and we're very close," she said.
The 43-year-old man was last seen driving away from his home in Mairangi Bay on the North Shore, on the 24th of February.
Sergeant Goldie says Mr Chance's phone data indicates he may have been in the Irwin Rd, Karaka area between 11pm and 12am on Sunday, February 24.
In terms of his health, his mother says Mr Chance was completely healthy, and there were no indicators he would disappear like this. She says she's tried everything possible to find her son.
"We've exhausted everything we know. It's just a mystery, I mean, it's a nightmare really," she told RNZ.
"He would never leave the things behind, he takes everything with him - he doesn't even have his toothbrush."
There has been no activity on Mr Chance's credit cards, and no sightings of him or his car, a distinctive red Nissan Skyline, since the 24th of February.
In a statement released on Tuesday, police confirmed he has not left the country.
His car was captured on CCTV at 4.17pm, going off the Drury off ramp on the Southern Motorway.
Police have been searching for Mr Chance, using ground crews and helicopters to try and locate his car.
His mother has an impassioned plea for the public.
"We would love the public, if they have dash cams to look at their dashcams from Sunday night, truckies, anyone who has a dashcam please, please have a look," Ms McGregor said.
She says she is "praying" her son will be found alive and well.
Police have reiterated this plea, asking anyone with a dashcam who was travelling on the Southern Motorway on Sunday, February 24 to contact police.
Read a full statement released on Tuesday night by Ms McGregor below:
"On Sunday 24th of February, my son, my boy, went missing.
He told his housemates he was popping out, but never returned and we have been desperately trying to contact him and find him ever since.
There have been many sleepless nights, and this past week has been hideous - a total emotional rollercoaster for our family.
But we are not giving up hope on finding him.
It's not like him at all not to be in touch with us and his friends.
He is so loved, obviously by us, his family, but also by his many friends.
He might look like a big tough guy, but he is an absolute softie, a gentle giant.
He's an animal lover and really likes to travel, is into fitness and has a wicked sense of humour.
He likes to play the clown and he adores his sister Ginny, who has travelled from the United States to help in the search for him.
We have been overwhelmed by the number of his friends and people who have offered to help find him.
People have been searching in bushes by the road in South Auckland, Waikato and Coromandel, checking the back of their properties and some people have even been searching in helicopters for his car.
We are doing everything to find our Denver.
We ask that anyone who has any information, no matter how small they think it is, that could help us find him to contact Police immediately."