Queensland murder victims 'were minding drugs'

Two men murdered in central Queensland were watching substantial drug crops for the man accused of killing them 25 years ago, police allege.

Robert Grayson and Derek Van Der Poel were last seen alive in Gladstone in May 1993 and following extensive investigations a 51-year-old man was charged with their murders and drug production on Monday.

"He has been a person of interest in this investigation for quite some time," Detective Inspector Darrin Shadlow told reporters.

Police allege the men were minding marijuana planted in Kroombit Tops National Park when they disappeared.

"We do have theories in relation to the cause of death and at this stage I'm not willing to disclose them," Det Insp Shadlow said.

The national park has been searched several times but the men's bodies have not been found.

So far, more than 100 people have been questioned and investigations into the alleged murders continue with police looking at whether another person was involved.

A $250,000 reward was offered for information which leads to a conviction over the murders.

The accused is due to front Gladstone Magistrates Court on Tuesday.