It has the hallmarks of an episode of The West Wing: the Prime Minister was given a rare and precious artefact. The artefact went missing. And now the staffer who was supposed to have been looking after the artefact is gone.

The ABC reported on Sunday that Dr Lisa Roberts, great-granddaughter of one of Australia’s most prominent artists, Tom Roberts, gave a valuable coin that had once belonged to the artist to the Prime Minister’s Office to be included in a public collection. It is alleged the coin was handed to Turnbull adviser Michael Napthali at the official opening of the National Gallery of Australia’s major retrospective exhibition of Roberts’ artwork on December 2, but it went missing shortly afterwards. The coin has not been accounted for since the event.

Napthali was moved from the Prime Minister’s Office to Arthur Sinodinos’ office after the incident, and Crikey understands Sinodinos expressed extreme displeasure over the coin incident. A spokesperson from the Prime Minister’s office told Crikey that Napthali had now resigned for family reasons.

The coin has been handed down in the Roberts family for generations, engraved with the letter T. It had a note that read “1st artist money TR”. Roberts says she handed the coin to Bruce Parncutt, former president of the National Gallery of Victoria’s council of trustees, and he says he passed it to Napthali.

The incident is now being investigated by the Australian Federal Police, and Roberts says she is determined to get to the bottom of the saga. “I’m not giving up,” she told Crikey. “I don’t suspect foul play, I just suspect sheer embarrassment.”

Roberts told the ABC that she wanted statutory declarations from the people involved.

The lost coin has already been investigated by the National Gallery of Australia, but it did not uncover what has happened to the coin. Roberts says she has had no contact from the Prime Minister’s Office since the news of the lost coin went public, and she was not aware that the coin was missing until she heard it from a third party.

“I just want them to be honest,” she told Crikey.

Michael Napthali was one of the key architects of George Brandis’ controversial National Program for Excellence in the Arts (NPEA), announced in last year’s budget. After Malcolm Turnbull became prime minister, Napthali moved to the PM’s office as an adviser on “arts, communications and intellectual property”. The NPEA was widely panned by the arts community, as it was set to favour big, commercially successful arts organisations over smaller community arts organisations. Crikey has previously reported that Napthali was closely involved in discussions between Brandis and major performing arts organisations and was an architect of the program, which was set to strip $105 million from the Australia Council.