Charities "will face a time of reckoning" after the coronavirus crisis, according to the NI Council for Voluntary Action (Nicva).
Its chief executive Seamus McAleavey told BBC News NI charities were "trying their best to cope" with an immediate loss of many fundraising streams.
He warned some could face a "slow uphill climb" after the pandemic.
The charity, St John Ambulance, is among those under threat, a volunteer said on Tuesday.
It was announced earlier this month that Northern Ireland would receive at least £10m in Treasury funding for the charitable sector.
St John Ambulance volunteer Matt Dempster said the loss of the service, which provides support at major events and runs first-aid courses, would have "a major impact on the sporting and community life of Northern Ireland".
The organisation has 1,860 volunteers in Northern Ireland and also has contracts with a number of health trusts.
St John Ambulance has started what it called an "urgent public appeal" to raise £100,000 to "support our continued commitment to combat the Covid-19 crisis".
Speaking on BBC Radio Ulster's Good Morning Ulster programme, Mr Dempster said: "We are at the point now when we are having to sit down very very carefully and study our books and study what we have left.
"We are one in four of those charities that do feel massively threatened by the lockdown of coronavirus.
"We are not saying that the lockdown should be loosened," Mr Dempster added, "but what we are saying is we need funding, we need government help here and we need public help.
"We need help to survive."
Jackie Trainor, a director at Northern Ireland Chest Heart and Stroke, said that charity estimates it could lose at least £1m of the £4m it needs to operate annually because of the pandemic.
"We provide care to over 4,000 people every year and we have about 50 staff delivering that care through support groups," she said.
"This could have a catastrophic effect; we will have to cut down on services and they are particularly for people who are vulnerable or highest risk to Covid-19 in terms of these conditions."
She said the charity has furloughed most of its fundraising team, but the care services team continues to provide help to users by phone or online.
Northern Ireland's Chief Charity Commissioner Nicole Lappin said: "Some charities may have been in difficulty before Covid, but certainly Covid will have added to that for them."
The postponement or cancellation of major events such as the Belfast City Marathon has limited fundraising opportunities for many organisations.
However, Ms Lappin believes the public has been "very altruistic" in its support for Covid-response causes, such as the BBC's Big Night In, The 2.6 Challenge and the efforts of Captain Tom Moore in raising tens of millions of pounds for health charities.
She did, however, encourage people "to consider the full range of charities that are out there".
Seamus McAleavey, from Nicva, said: "The real losers in all this are the clients and the patients. Some of these charities take up a lot of slack within the system, so if they do go under, more and more people are going to have to turn to the public sector."
He described the additional funding for the charities as a "significant amount", but "it will not be enough to cover all the problems".
"The sooner they can start to roll out some of this emergency funding the better.
"It is to replace, at least part, the fundraising income which has now disappeared," he added.