Excess Baggage for portable stroller in carry-on

Hi, we were travelling from Melbourne to Gold Coast and back as a group of 6 Adults, 2 toddlers and 1 infant. We were travelling first time after Covid for a holiday with our infant who had never been on a flight. After a pleasant and tiring trip we were just trying to get back home. We had checked all our lugged prior to boarding and ensured we were below weight. Our combined allowed weight was 56kg (7kg * 8) and our total baggage was 54Kg. We were weight-checked by ground staff at the gate before boarding the flight from GoldCoast to Melbourne. They advised we need to have everything weighed and we had no problems we were confident our luggage was under. They weighed everything, including water bottles, food etc. we were still under weight than our allowed allowance. However, when they noticed we have no extra luggage, they asked me to also weigh my portable stroller which put us over and we were asked to pay $70 for it. They advised it would've been free weight if it was dropped at the check-in counter. We tried to reason with them a portable stroller is usually always allowed. How will an infant relax at the gate until we board the flight. Also, if we check-in or carry-on it would make no difference to a plane's weight. We did not have this issue when boarding from Melbourne. We are only carrying back what we got from Melbourne, if anything, we were travelling back home with less luggage as we had used up things like nappes/baby food etc. They threatened me at the gate if i argue with them or don't pay, i might not be allowed to board the flight. When we arrived at Melbourne Airport, we spoke to Teresa from Jetstar who met us at the gate and explained our situation. She agreed with us that we were wrongly charged. She also confirmed the same with her team leader Mine who advised the same. We even requested them to weigh all our baggage as soon as we disembarked from the plane as we had nothing to hide. They re-iterated to me that portable stroller in a carry case is always allowed at Melbourne Airport as infants cannot make the long walk from security till the gate. We spoke to Aryanna in customer service team, and they advised there is nothing they can do and she cared very little.

Australia-Gold Coast
Australia-Melbourne - Tullamarine