Monarto and TWPZ must hold almost half the regions giraffe population currently?
Close enough! Both of those facilities hold around 20 giraffes at any given time, so considering the regional population is around 110, this would be a little under 50%.
Mogo and Australia Zoo both have reasonable sized breeding herds - with Australia Zoo holding multiple breeding cows that produce calves every 18-24 months. Even with these calves been transferred out before the age of 2-3 years, that would take them close to double figures.