District health boards in the Wellington region have found 100,000 masks in a deteriorated state.
There are 90,000 masks with corrupted elastic at Capital and District Coast DHB and a further 10,000 at Hutt Valley DHB.
Incident controller Joy Farley told the Herald in a statement the masks were purchased 10 years ago and equated to about 10 per cent of total masks currently held by the region's three DHBs.
"We would reiterate that the 100,000 masks are in the process of having elastic replaced and will be fit for use in non-clinical areas, and that there have been no issues or problems with masks provided by the Ministry of Health."
The Herald requested an interview with Farley to explain the circumstances surrounding the deterioration and how the masks were being fixed, but a DHB spokesman said they had nothing further to add to their statement.
The Association of Salaried Medical Specialists executive director Sarah Dalton said the situation raised questions regarding why the stock was left to sit around for 10 years and not be used.
"You'd like to think that rather than having backlogs and stockpiles you keep things ticking over so you can get use out of that stuff", she said.
It was possible other DHBs around the country could also be sitting on expired stock, she said.
But Dalton was glad the repaired masks would only be used in non-clinical areas.
"They [the DHBs] have clearly decided to dig out what is effectively expired stock for use in what is essentially low risk settings and not for front-line clinical work.
"It's a sensible thing to properly prioritise the fresh and current stock where it's most needed."
Otago University Clothing and Textiles Centre director professor Raechel Laing said there were two potential factors in the deterioration, those being the composition of the elastic and storage.
Depending on how the masks were stored, different elements could have contributed to breaking down the elastic, she said.
"The exposure to light, it could be artificial light or it could be outside light, or exposure to variations in heat and in moisture", she said.
Laing said the masks wouldn't have been manufactured to be particularly resistant over time because they were designed to be disposable.
DHB-held reserve supplies are not managed by the Ministry of Health but a spokesperson confirmed the Ministry was aware elastic in masks from some DHB stores had perished.
They said the Ministry continued to work with both the health and non-health sectors to ensure that PPE was available to those who needed it.
"We currently have good supplies of PPE and will distribute these to ensure that workers in a range of organisations, including disability support, home care and support workers, NGOs, pharmacy and other providers have the protection they need."