In Aust the min bmi seems to 35. Some surgeons will operate on patients with a slightly lower bmi but they must have severe health complications (high blood pressure, heart problems, diabetes, etc.).
It‘s not common for people to lose so much weight after surgery they attain a bmi in the low end of the healthy range or lower & stay there. Those that do drop to a bmi of 18 or 19 more often or not gain some again after because it’s difficult to maintain long term. Life kicks in, your hunger increases & metabolism slows again.
The average weight loss after surgery is about 65% of the weight you need to lose to put you in the upper end of the healthy range after 5yrs. There are a couple of people on this forum who struggle to maintain & have a low bmi. I’m sort of one of those people.
If I miss a meal or a snack I can drop weight. Do it a couple of times in a week & the drop sticks. Consequently, I’m very routine in my diet of when & what I eat - sometimes I feel like I eat all day which is challenging when I’m often not hungry. (Honestly, my GP & surgeon would like me to put on a couple of kilograms but I’ve dropped a kilo in the last 2 - 3 months so I’m a bit nervous to go back next month.)
What is your goal weight?
As with all things in life if you put in the hard work & are committed to your plan (very careful with your diet & very active) you can attain your goals & maintain it. However balance is key. You have to be able to live the life you want at a weight you are happy with.
I hope I don’t sound negative. Your concern is legitimate. I suggest you speak with your surgeon & nutritionalist about your worries.
Good luck.