The woman, believed to be from the UK, said she had never left the centre any later than 6.29pm — still within the time frame she was paying for.
But she said she was left feeling "confused and embarrassed" after the centre manager took her aside recently to discuss collection times.
In a follow-up post, the mother explained that on days when she picked up her daughter earlier, she had often witnessed staff leaving early, well before 6.30pm.
"She said she wanted to remind me that her staff finish at 6.30pm as that is all they are paid until," the woman posted about the nasty exchange.
"She said she has never had to consider introducing a "late" fee before but might have to unless I'm more punctual.
"I double checked that their clock is the same time as my watch/phone clock ... She said that her staff report I didn't actually leave until 6.29pm the other night, and they ended up having to stay until 6.45pm to do all the last-minute jobs they couldn't do while [the child] was there."
While the busy mum reminded the staffer she had specifically chosen the centre because she needed care past 6pm, the conversation ended "rather awkwardly", with the manager saying all parents would be issued with a letter about lateness and late fees.
"Surely this is more of a managerial issue than a parent one? I pay for childcare up until 6.30pm," the mother wrote.
"I'm obviously going to be contesting any late fees added on to my bill since I'm always out of there by 6.30pm.
"I might also suggest they do introduce a sign-out book so it's documented what time I've collected [my daughter]."
The woman was inundated by support from fellow parents who condemned the centre manager's attitude.
One parent posted: "Cheeky buggers — if you've paid until 6.30 then you should be allowed to stay until 6.30. They need to plan their staffing accordingly with shifts ending later to allow for any closing down procedure if necessary but that's not your responsibility."
Another wrote: "Very weird. Unheard of in all my experience. That manager is delusional if she thinks her request appropriate or even rational!"
And another added: "Then they need to sort out their advertising, decide exactly what time they offer childcare until and arrange their staff appropriately. If you have paid until 6.30 you should be able to pick up at 6.30."