Forensic officers have begun excavating wetlands north-west of Hobart, determined to unearth a trace of missing woman Lucille Butterworth.

Miss Butterworth, a 20-year-old typist and beauty queen, disappeared from a Claremont bus stop in August 1969.

Police say there is credible evidence that Miss Butterworth was murdered by a passing motorist, who then dumped her body alongside the Derwent River between Granton and New Norfolk.

John Butterworth was just 15 when his sister vanished.

Speaking from Darwin on Monday, Mr Butterworth said he was feeling anxious, nervous and frustrated as police started clearing dense vegetation at a disused, overgrown car park where they believe her body lies.

Mr Butterworth said he was in constant contact with police for news on their progress.

‘‘Forty-six years is a long time but there’s no such thing as a perfect crime,’’ Mr Butterworth said.

‘‘We just really hope they find something to bring our family some closure.

‘‘But it’ll never be closed until the perpetrator has been charged,’’ he said.

Mr Butterworth said he had been deprived of the opportunity to get to know his sister properly, and left with a lifetime of uncertainty about what exactly happened to her.

Inspector David Plumpton said police would begin methodically sifting through tonnes of dirt once the area was levelled.

‘‘At the end of the day it relies on just constant grinding, hard labour,’’ Inspector Plumpton said.

‘‘Bit by bit, as you go into it, you realise it is quite a momentous task.

‘‘However, if there is something there, I’ve got no double we’ve got the people and the equipment to find it,’’ he said.

The excavation could take at least two weeks.

Inspector Plumpton said any evidence found may contribute to police pressing murder charges against a person of interest.

‘‘But first and foremost, at the moment, it is about providing the Butterworth family with some opportunity to bury Lucille,’’ he said.

A coronial inquest into Miss Butterworth’s murder is scheduled to start on August 31.