Josh Gonzales, a 30-year-old secondary teacher from NSW's Illawarra region, said digging into personal time was crucial to managing the workload at his public school.
He grew close to dumping his profession altogether at least "a couple of times on the bad days" and told AAP many others have confided similar struggles.
"The amount of teachers I've spoken to where they break down in front of me, it's like how can I do this? What can I do to make things different?" he said.
The NSW Teachers Federation on Wednesday will argue their allocated preparative hours are far too short, armed with fresh polling.
After-hours workloads could prompt statewide exits from education and contribute to new teacher shortages, they will say.
Primary teachers receive two hours a week - 24 minutes a day - to speak to parents, write up disciplinary reports and amend lesson plans for students with additional needs, they say.
A survey of 1400 union members found about 85 per cent of the state's teachers, like Mr Gonzales, considered leaving education because of shortfalls.
Securing additional allocated hours would incentivise a similar proportion to stay, according to the survey.
In written responses, teachers said the hours would allow them to better plan lessons, especially for the apparently increasing numbers of students with special learning needs.
Nearly all respondents said they felt complex requirements among students had grown since they began teaching.
NSW Teachers Federation president Henry Rajendra said the results could not be ignored.
"Teachers are telling us in overwhelming numbers that they need more prep time to … properly attend to the needs of kids with learning difficulties as well as those who need to be extended," he said.
Two further hours each week would alleviate many teachers' need to work from home, the federation will argue with an open letter outside state parliament.
Mr Gonzales - a local federation representative with a decade in education and three years' teaching experience - said friends and students' parents were often unaware of his out-of-class workload.
Employers and the department "expected" it.
"I wouldn't say there's too much appreciation (because) it's normalised behaviour … you definitely feel undervalued," he said.
Now the father of a newborn girl, Mr Gonzales said he and many others in the profession were torn for their compassion.
"Before, I was happy to go home and do a bit of work if it means a student's going to succeed in the classroom, (but) I shouldn't have to choose between caring for my family and getting the work done," he said.
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