St Vincent de Paul Society NSW chief executive Jack de Groot said there is an "urgent need" for more volunteers to help run its shops in the lead up to Christmas.
The shops help raise funds for people who are experiencing homelessness, domestic and family violence and poverty.
"Demand for our services is increasing at the same time lower than usual numbers of volunteers is impacting our ability to keep all of our shops open, so that they can raise crucial funds to pay for them," Mr de Groot said.
"Vinnies Shops raise critical funds for the Society every year, which it relies on to help people at risk of or experiencing homelessness, domestic violence, and more.
"Every day, we provide food, accommodation, casework, help with bills, and other daily necessities to people in need.
"Each year, our members and staff provide assistance to tens of thousands of people, we serve thousands of meals, give millions in direct material assistance, help thousands of women and children escape domestic violence - the list goes on.
"We are able to do this in great part due to the revenue that comes from Vinnies Shops. We have an urgent need for more volunteers or our ability to help people in need may be restricted."
Mr de Groot said there were "significant pressures" on Vinnies' services that are driving demand up.
"In the first quarter of this financial year, there was a 20 per cent increase in the number of people seeking assistance from our DV services," he said.
"We are at a point where emergency COVID unemployment payments are being withdrawn and those people will lose the safety net that was keeping them housed and fed."
Brooke Simmons, St Vincent de Paul Society NSW vice-president for Social Justice, said volunteering at a Vinnies Shop is "one of the most rewarding things you can do."
"Money is tight for a lot of people right now due to COVID but if you've got some time spare, I can't recommend volunteering with Vinnies enough," she said.
"Some of our volunteers have been with us for decades and they do it because they enjoy the company of like-minded people they find themselves with.
"Sharing collective goals with great people to raise money for people who need help - it doesn't get much better than that."
Vinnies Shops are located at Fairfield, Fairfield Heights, Mount Pritchard, Liverpool and Miller.
- If you are interested in volunteering with Vinnies Shops anywhere in NSW, please call 02 9350 9600 or register your interest at vinnies.org.au/shopvolunteernsw.