“Can you explain to her (a potential trial witness) it’s not in her best interest to show up at trial,” Wagga man Joshua Clifton Chalmers said to a friend on the other end of the line.

Fearing he would be jailed for six to eight years for his bungled attempt to rob the Tolland newsagency with a knife in March last year, Chalmers tried desperately in a phone call and letters from behind bars to influence people either to not turn up at trial or give him an alibi.

He even told one man in a letter to say to one of the alleged victims Chalmers would give him $20,000 to say it was not Chalmers who tried to rob him.

Chalmers’ counsel, Michael King, downplayed the seriousness of his client’s four attempts to pervert the course of justice, saying it was Chalmers “hanging on to a bit of old bravado”.

But Crown solicitor, Lisa Hanshaw, told Wagga District Court on Thursday the phone call “posed a risk of physical or mental harm” to the witness that luckily came to nothing because the person Chalmers spoke to did not act on his request.

Both Mr King and Judge Gordon Lerve noted all jail phone calls and mail were monitored by staff.

“While any public justice offence is serious … these actions by the accused were amateurish in the extreme and were doomed to failure, and that would have been obvious to the accused had he given it a moment’s thought,” Judge Lerve said.

The court heard Chalmers did not make further attempts to influence witnesses and later pleaded guilty the charge of assault with intent to rob while armed.

He was jailed until March, 2017, for that offence.

Judge Lerve gave Chalmers a head sentence of four years, with two years non-parole backdated to December 23, last year, for four counts of trying to pervert the course of justice.

In effect, Chalmers will spend nine months longer in prison.