Thousands of health and education workers have been criticised by Prime Minister David Cameron after they walked out on strike in protest against frontline budget cuts.
The walkout on Wednesday affected all health workers except doctors, and some school staff.
Unison regional secretary Patricia McKeown said: "The workers are facing some of the worst cuts in their history.
"They have not been well treated or well served by the Government and that is contemptible."
But Mr Cameron told the Conservative party's annual conference in Manchester that "real fairness" is not just about what the state spends, but about the link between what people put in and what they get out.
"As we debate what people get from the state, let's remember how we generate taxes. So to the unions planning to strike over public sector pensions I say this: You have every right to protest, but our population is ageing. Our public sector pensions system is unaffordable.
"The only way to give public sector workers a decent, sustainable pensions system, and do right by the taxpayer, is to ask public servants to work a little longer and contribute a little more. That is fair.
"What is not fair, what is not right, is going on strikes that will hurt the very people who help pay for your pensions."
Stormont health minister Edwin Poots had earlier said he was disappointed after staff voted for industrial action, which public service union Unison blamed on frontline cuts.
The education and health departments are competing for funds amid a wider public spending squeeze. The education budget is facing a shortfall of £300 million over four years, while health is facing cuts of £2.3 billion.