"I had a tearful call from her this morning saying she would be on a later flight. It's really disappointing," Mrs Walsh told news.com.au.
"She's got a short time in the morning to see her sister-in-law in the morning now and then she'll be back on a plane tomorrow to fly home. She'll be so upset, it will absolutely throw her right off."
The family are worried about how the disruption will affect Aunt Beryl as, though she is in good form for her age, travelling long distances can be taxing on her health.
"This was the last time she was going to fly up, she said 'I'm not sure I can do this again'," Mrs Walsh said.
She said she'd initially thought the 11.50am flight may have been among a string of services cancelled due to windy conditions in Sydney.
But when the flight arrived on schedule at Port Macquarie, Mrs Walsh demanded an explanation from Qantas ground crew, who explained that the flight had been downgraded to a smaller plane - meaning it was now overbooked.
She was irate that her elderly aunt had been bumped off her flight after she had informed Qantas the night before of her situation.
"They called me asking if she could be put on a later flight and I said 'no' because she only has one day in Port Macquarie," Mrs Walsh said.
And she said her plea for a Qantas staff member to check on her aunt in Sydney fell on deaf ears.
"I asked if someone could call the gate to make sure she is okay, but apparently it's 'chaos' down there," she said, demanding an apology.
Aunt Beryl had flown with Qantas all her life and was a frequent flyer member, she said.
"She always says 'I only fly Qantas, it's the best airline, it's taken such good care of me over the years'. Now this!"
A spokesman for the airline said Qantas had been in contact with Aunt Beryl's family and was "looking to see if we can change her flights home so she can stay longer in Port Macquarie".
"We had to swap out a larger aircraft for a smaller one which meant Beryl travelled on a later flight," the spokesman said in a statement.
"Due to the strong winds in Sydney we've had to cancel flights and some of our aircraft are not in the right ports, which has caused some disruption to the schedule.
"We're working hard to get everything back on track as soon as possible and we thank our passengers for their patience."