Peter James Saunders, 26, was sentenced for spitting on the ACT police officer during an arrest.

The spit landed on the officer's face and on her clothing.

Saunders was charged with four offences; common assault for the spitting, resisting a public official, minor theft, and a failure to appear in court.

Magistrate Peter Morrison sentenced him in June to a total of nine months imprisonment, which included seven months for spitting on the officer.

That sentence would not have seen Saunders freed until New Year's Eve this year.

But Saunders appealed the sentence in the ACT Supreme Court, which handed down its decision late last month.

Justice John Burns found the seven month sentence for the common assault was manifestly excessive.

"I am satisfied, on the basis of the objective circumstances, that the Magistrate has overstated the objective seriousness of the offending," he said.

Saunders' lawyers pointed to a number of other similar cases, in which offenders who spat on police and security guards were given much lighter sentences.

Justice Burns reduced the common assault sentence to four months, meaning the new total sentence for all four offences was seven months.

That sentence expired on October 31.

Justice Burns acknowledged that Saunders, at the time of his sentencing, showed little hope of rehabilitation.

He also acknowledged the sentencing decision was a discretionary judgment by the magistrate.

"I am mindful of the undesirability of being seen to tinker with sentences imposed in the Magistrates Court," he wrote.

"That, however, is not a reason why, if I am satisfied that the sentence imposed with respect to the common assault matter is manifestly excessive, I should not intervene and vary that sentence."