"The numbers will go up and down, and we know that when there's mixing and (the) introduction of new social networks," Chief Health Officer Kerry Chant said on Wednesday.
A surge in cases can be controlled by the community's actions, including getting vaccinated, and parents having booster doses, she said.
Dr Chant urged people with school-aged children to "think about you and your extended family" before the return to classrooms.
"I can't stress enough the urgency. We know that booster will increase your level of protection," she said.
The booster plea comes as the state begins shipments of 24 million rapid antigen tests as part of a back-to-school program that will require students to be tested twice a week.
"Schools will be back day one, term one for students, and we are not moving from that, so everyone has to be prepared," Education Minister Sarah Mitchell told the Seven Network on Wednesday.
Queensland has delayed the beginning of term one due to concerns over Omicron.
Ms Mitchell said reclassifying teachers as essential workers was an important step in ensuring schools stayed open.
Teachers who are close contacts will instead be given rapid tests, and will continue to work if they are well.
The Opposition repeated calls on Wednesday for the government to use public schools as vaccination hubs.
"We've had them in place for decades, distributing vaccines to public school kids," Labor leader Chris Minns said.
"It's a great and efficient way of distributing these vaccines. It should be considered by the NSW government."
The government announced on Wednesday the waiting period between second vaccine doses and boosters had been shortened, opening up eligibility for an additional 1.8 million people.
Adults who were vaccinated with their second dose three or more months ago are now eligible for a booster. The waiting period was previously four months.
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