A woman who was behind bars with two of her four children has been released with two years of her sentence still to run.

Alana Hayley Cairns, 26, was jailed last July for three years after pleading guilty to an aggravated robbery in which she acted as a lookout for her partner, Joshua Smith, and his mother, Joanne Smith.

Using a knife to threaten a staff member, they made off with $13,728 from the Law Courts Hotel.

It was Cairns' first conviction.

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At sentencing, the court heard she would be housed in the Mothers With Babies Unit at Christchurch Women's Prison with her baby and another which was due in January.

But her stay behind bars came to an end a third of the way through her sentence - the earliest available opportunity for parole.

The Parole Board noted Cairns was a minimum-security prisoner, described as "polite and compliant", and had completed all appropriate rehabilitative programmes.

Panel convener Jim Thomson said she was not a risk to the public.

On December 22, 2017, Cairns had been instructed to play the pokies while reporting back to the Smiths about how many staff were present. She booked a room at their request, so she could obtain the PIN code to the external door for after-hours access.

Shortly before midnight, when the Smiths arrived, both in dark hoodies, both masked, the door was open.

They hid under a table in the restaurant, waiting for the lone staff member before Joshua Smith brandished a knife as they demanded she empty the safe.

They sent the victim into the toilets while they made their escape.

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Cairns and her partner were found by police the next day with some of the stolen cash.

Joshua Smith and his mother - who were also convicted of a prior robbery of a Christchurch bar - were sentenced to five years and six years three months respectively.

"The relationship which Ms Cairns had with the co-offender was chaotic and constantly distressing because of his behaviour and it required frequent police callouts," the Parole Board noted.

"This may have had a significant influence on her becoming involved in the offending."

Because of her "selection of unsuitable partners", the board imposed a condition of release that Cairns inform Probation about the commencement of any intimate relationship.

Other conditions included:

• Not to communicate with co-offenders.
• To attend an assessment and complete any programmes or counselling as directed by Probation.
• To live at an approved address.
• Not to contact any victims.
• To attend a reintegration meeting as directed.