Teachers planned to march from Sydney's Town Hall to the head office of the Catholic Archdiocese of Sydney on Friday, one of several rallies taking place around NSW and Canberra.
Newcastle, Wollongong, Bathurst, Dubbo, Lennox Head, Port Macquarie, Tamworth and Wagga Wagga also held rallies on Friday morning.
It's the first strike by Catholic schoolteachers since 2004.
"Uncompetitive salaries, unsustainable workloads and crippling staff shortages have pushed them beyond their limits," Independent Education Union secretary Mark Northam said.
The strikes come after public schoolteachers went on strike twice in the past six months.
Both groups want increased wages, more time to plan classes, less paperwork and an end to staff shortages.
Catholic teachers are seeking a raise of up to 15 per cent over two years, compared with an increase of up to 7.5 per cent that public schoolteachers are chasing.
Public schoolteachers have had wage increases capped at 2.5 per cent a year since 2011, and while Catholic employers are not bound by the cap, they typically take their cue from it.
The IEU says it's eagerly awaiting the budget to see if Premier Dominic Perrottet's hinted changes to the cap materialise.
"I can't guarantee that everyone will be happy with where we land ... but we have the competing interests of the budget to put record amounts in schools, in education right across the board, as well as in health care, as well as in public transport," Mr Perrottet said on Friday.
"We have to balance that all up and make the decision."
Education Minister Sarah Mitchell said the strike was disappointing because it inconvenienced parents and students, and the government was already working to address teacher concerns.
"We are trying to support our workforce when it comes to decreasing that admin burden and the workload," she said.
"We are going through our wages policy as part of the budget."
In a letter on Tuesday, the union told parents the strike was in the best interests of them and their children.
Improving school working conditions helped improve children's learning conditions, Mr Northam wrote.
Catholic Education, which has 64 schools and preschools in the dioceses of Canberra and Goulburn, supported pay increases but said the strike action was a disappointment.
Teachers were given a 2.04 per cent pay increase from January, Catholic Education director Ross Fox said.
The dioceses had also agreed to at least match any pay increases for public schoolteachers, lift pay for support staff to match the state system, provide an extra day of planning, and address workload concerns.
"The reality is any costs outside of government funding have to be met by our parents so the challenge always is providing our staff the best pay and conditions we can while keeping our education affordable for the families we seek to serve," Mr Fox said.
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