The union representing prison staff is warning the loss of dozens of workers who refused to get vaccinated will disrupt prison life.
The Department of Corrections insisted there was no problem, but the Corrections Association said programmes such as literacy courses were at risk in some prisons.
Under the Government's Covid-19 vaccine mandate, prison staff had until early December to be fully vaccinated.
Since then, 78 people, 1.25 per cent of prison staff, have lost their jobs because they would not get vaccinated, while 6088 are fully vaccinated.
"There's staff that have got the vaccine and feel like they have been blackmailed into that purely because of the speed in which it was done."
Du Plessis said job losses were spread across New Zealand's prisons, but absences would make more work for other staff.
"Losing Corrections staff isn't just a case of hiring someone else. It's a tight labour market.
"It's a struggle to hire and then you factor in the fact it takes six months to get someone up to full proficiency and able to be a Corrections officer.
Rimutaka Prison has lost eight Corrections staff because they did not get the Covid-19 vaccine. Photo / Richard Tindiller, RNZ
"It's quite an extensive process to replace those staff that we've lost."
Spring Hill prison in Waikato lost 12 staff, the most of any jail, while Northland Region Corrections Facility lost nine, Rimutaka eight, and Tongariro and Christchurch Men's seven.
Du Plessis said the losses would be felt at the most affected sites. They could force prisons to postpone programmes for prisoners, such as literacy, numeracy or skills courses, until more staff were hired.
In a statement, Corrections chief custodial officer Neil Beales said the vaccine mandate had no significant effect on staffing, or impact on the safe and secure operation of prisons.
"This was because the vast majority of our prison-based staff were already vaccinated or received their vaccination ahead of the required deadlines."
Since March 2020, the department had planned to manage the risks associated with Covid-19.
"As part of this, we have planned for how we can safely operate at reduced staffing levels in the event significant numbers of staff are unwell or need to isolate, and we are well prepared to operate with reduced staffing levels," Beales said.
New Zealand Howard League chief executive Mike Williams said he doubted the loss of fewer than 80 staff would have much effect. But they would be missed if they were in specialised jobs.
"It's absolutely the right thing to try and keep Covid out of the jails and I think that's probably one of the reasons why you can't have unvaccinated people working in the jails, because they are more likely to catch particularly the Omicron variant and pass it on."
The league, which advocates for penal reform, was not able to get volunteers into all but two prisons because of rules aimed at preventing Covid-19.
Williams said the falling prison muster would also help ease pressure on staff.
In December, about 7700 people were behind bars, well down on more than 10,000 in early 2020.
Beales said nine staff were on unpaid or annual leave while they went through their "vaccination plan". Most of them were having the AstraZeneca injections, which had only recently become available to them.
Another 23 staff on long-term leave were yet to declare their vaccination statuses.
"If they have not already done so, they will need to complete their primary Covid-19 vaccination course before returning to work," Beales said.
