Matthew John Millard had his total jail term reduced by eight months on Tuesday, while he will now be eligible for parole six months earlier than expected.

Millard, 37, was arrested in July 2018 after encouraging his pit bull to maul a female constable, who continues to suffer "significant disability" and psychological trauma as a result of the attack.

The incident took place when the constable knocked on Millard's door in Rivett to question him about an elderly woman's mobility scooter, which he had stolen in Weston four days earlier.

The day after his arrest, Millard unsuccessfully applied for bail in the ACT Magistrates Court. While in the courtroom, he told a different police officer: "C..., yeah, you, I'm gonna kill you, you mole." Millard then made "pew-pew-pew" noises while pretending to shoot the officer with his fingers. As he was escorted out, he repeated the threat and feigned gunfire towards several other officers.

Millard eventually pleaded guilty to a string of charges linked to each of the incidents.

In the ACT Supreme Court last year, Justice John Burns described Millard as "a thoroughly dishonest person" while sentencing him to four years and nine months in jail over the theft and dog attack.

Millard then appeared again in the Magistrates Court, where Chief Magistrate Lorraine Walker handed him a further 16 months behind bars for threatening a reprisal against the police officers in court.

This took Millard's total jail sentence to six years and one month, and made him eligible for parole in September 2022.

Earlier this year, he failed in challenging the sentence imposed by Justice Burns.

But in the ACT Supreme Court on Tuesday, he successfully appealed against the penalty handed down by Ms Walker.

Millard's lawyer, Jane Campbell, conceded that Millard's behaviour on the day of his bail application was unacceptable.

But she said he had acted on impulse and out of "self-pity and frustration" after his offending landed him in a courtroom packed with police officers and the media.

Ms Campbell said Millard, in custody and suffering from physical disabilities, had no capacity to carry out the shooting threats and was simply trying to provoke a reaction.

She argued that Ms Walker should have made the sentence of 16 months at least partially concurrent on the sentence imposed by Justice Burns.

But prosecutor Marcus Dyason disagreed, saying it should remain wholly cumulative because the threats Millard made in court involved "separate and distinct criminality" from the scooter theft and the dog attack.

Justice Michael Elkaim ultimately ordered that half of the 16-month sentence be served concurrently with the sentence imposed by Justice Burns, reducing Millard's total jail term to five years and five months.

Justice Elkaim also reset Millard's non-parole period, cutting it by six months. Millard will now be eligible for release in March 2022.