Detective Inspector Steve Benson said on Monday that investigators were following "strong lines of inquiry" and police hoped the reward money would "entice those people in the community that still have information" about the suspected murders to break their silence.

Ms Hickie was 18 years old when she was last seen on the Pacific Highway at Belmont at about 7.15pm on April 7, 1979.

A fortnight later, on April 21, 14-year-old Amanda vanished after attending a dance at a high school in Gateshead - she was last seen on Lake Road at Swansea.

Strike Force Arapaima was formed in 2019 to re-investigate the suspected murders of Ms Hickie, Amanda and another teenager, 16-year-old Gordana Kotevksi, who was snatched from a suburban Charlestown street in 1994.

The reward money announced on Monday did not apply to Gordana's case. Detective Inspector Benson said there was a separate line of inquiry into her suspected death, but the case remained under the umbrella of Strike Force Arapaima.

"The disappearances of these teenagers triggered every parent's worst nightmare. The Hickie and Robinson families have been fighting for the truth for four decades, they deserve to know what happened to their girls," Detective Inspector Benson said.

"We're hoping this reward will lead to answers and potentially closure for those families.

"We're working to get an arrest."

Detective Inspector Benson would not comment possible persons of interest.

Police remain interested in a green mid-1970s four-door Holden Torana sedan, which they suspect was in the same area as Amanda when she was last seen four decades ago.

NSW Police and Emergency Services Minister David Elliott said the reward money showed the government's strong support for the detectives investigating the cases.

"For far too long, Robyn and Amanda's families have lived with the pain of losing a child but without any idea of how it happened or who is responsible," Mr Elliott said.

"They have been in limbo for four decades not knowing. They deserve answers now and we believe that offering these new rewards should be sufficient incentive for those with information to come forward.

"If your conscience won't make you act, maybe $2 million will."

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