Her body was found about 2.30pm approximately 10km away in the back seat of an abandoned purple Holden Commodore near a place known as Meeting of the Waters.
Detective Senior Sergeant Grant Coward said police were appealing for any sightings of "a distinctive purple 1987 Holden Commodore sedan with mag wheels'' driving around New Plymouth on Saturday afternoon.
"More specifically we are looking for sightings in the location of the top end of Frankley Rd, Carrington Rd, Baker Rd, Mangorei, Junction Rd and the Meeting of the Waters,'' he said.
Anyone with information was asked to call police on 06 759 9456 or 06 759 5500.
Ms McCullough's funeral will be held in New Plymouth tomorrow.