A Fire and Emergency spokesperson would not comment on the incident and directed the Herald to the prison.
Police did not attend the call-out and directed questions to Fire and Emergency.
Corrections say there was no threat to the wider security of the prison, but other prisoners in the unit were evacuated due to the smoke.
"Once the smoke has cleared, the prisoners will be returned to their cells."
Two staff members have been taken off site for a medical check and Health Services at the prison will check on the other prisoners' wellbeing.
Last week the Herald reported that tempers frayed inside the Waikato prison as it remained in level 3 restrictions.
Spring Hill, in the Waikato, is the only prison in the country at Covid alert level 3, with all other Corrections facilities operating at level 2 or 4, in line with the region's setting.
Spring Hill Corrections Facility prison director Scott Walker said the decision was made because of the site's proximity to Auckland and the "significant" number of staff who travelled from Auckland to the prison every day.
But the decision is taking its toll on prisoners, the partner of one inmate told the Herald.
She said tempers among prisoners were becoming frayed with limited time out of cells under the extended lockdown regime.
Last month a Corrections officer at Spring Hill Prison tested positive for Covid-19.
The unit of 120 men where the Corrections officer worked were placed into quarantine and tested while 14 men recently released and 23 staff were asked to self-isolate and get tested.