Melissa Trussell and her mother Rosemary Brown, 33, were last seen leaving their South Australian home at Blair Athol about 2.30am on May 13, 2000.
Rosemary's body was located in the Garden Island mangroves weeks later but Melissa's body was never found.
Police launched a fresh search on Friday in the Port River estuary at Garden Island in the hope of uncovering Melissa's remains to provide her family with closure.
They had a breakthrough when locating a vehicle that could be linked to the case.
A section of the front grille was retrieved which could help with identification of the vehicle.
Officers are investigating whether the vehicle is connected to the murder of the mother and the suspected killing of the teenager.
Attempts might be made to recover the vehicle for further examination with the search to continue for another day, police said.
Divers scoured a previously unsearched area of water and mangroves on the island's northern side.
"This search really does show to everyone that we never give up trying to find answers to those investigations," Detective Superintendent Darren Fielke told reporters.
Det Fielke did not know why the area near Garden Island was not searched 25 years ago but a cold case review identified it as a place for further investigation.
A number of exhibits obtained in the initial investigation have been resubmitted for forensic testing to see if modern technology might result in new leads.
Police believe Melissa and her mother were murdered in 2000, but no one has been charged, or any motive revealed.
Det Fielke hopes someone would come forward and explain what happened to the pair.
"Twenty-five years is a long time, a lot of water goes under the bridge, people's allegiances change over time, people forget about it, some people don't forget about it," he said.
"We are always in there prodding and poking away trying to get a breakthrough."
Melissa's father Barry Trussell earlier in July issued a plea for information to help provide closure on the 25th anniversary of Rosemary's body being found.
"A parent shouldn't outlive their child … and for a long time Melissa's disappearance has messed me up," he said.
"I'm hoping some fresh information will finally come to light to help police solve the case."
A reward of $1 million is available to anyone who provides information leading to the arrest of a suspect in Melissa's murder or the recovery of her remains.
Up to $200,000 is available to anyone with information that leads to the arrest or conviction of those responsible for Rosemary's murder.
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