Western Bay of Plenty area commander Inspector Clifford Paxton said the car had been travelling north towards Katikati and went off the road on a left-hand curve on State Highway 2 in Whakamarama.
The car was found down a steep embankment on the other side of a stand of trees, hiding it from view from the road.
Police said Miss Sinclair left home on Sunday to go to Mount Maunganui and was last seen leaving a friend's house in Katikati about 11.20 that night.
According to a colleague at The Porch bar in Waihi Beach, who did not want her name used, Miss Sinclair worked during the day on Sunday and had planned to attend a staff meeting at 3pm on Monday but did not show up.
Di King, a friend of the family, had known Miss Sinclair for about 20 years and said she was an ambitious, hard working young woman who was well liked and respected.
"She was strong-minded and one of those people that wouldn't be pushed around by anyone, but always conscientious. Her work ethic was just amazing and with the public she was like a PR person."
Mrs King said it had been out of character for Miss Sinclair to stay out of contact with her mother for so long, which had worried the tight-knit Waihi Beach community.
"She would always tell her mum where she was going. Her mum was her best friend rather than a mother and daughter relationship."
Miss Sinclair had grown up in the area and gone to Katikati College but had been living in Auckland in recent years, Mrs King said.
The young woman had moved back to Waihi Beach just before Christmas and had been working a number of hospitality jobs.
Vanessa Choat recently began working with Miss Sinclair.
She said it had worried people when Miss Sinclair did not show up to work on Monday without making contact with anyone.
A serious crash investigation was under way and police would like to hear from anyone who saw Miss Sinclair's blue 1997 Audi A3 between late Sunday night and yesterday.
