Designer Stubble:
"So growth of the organization is not keeping up with the population growth."
In all fairness that might be because of immigration combined with a low birth rate amongst Australians. If immigrants have a proportionally low number of JWs then they're not replenishing their numbers through that type of increase.
But why would Witnesses not likely be immigrants? Because of the disproportionate amount of poverty in that religion. Poor people, who have no skills or anything to offer to the country, aren't likely to be accepted by picky Immigration officers.
There is also a disproportionate number of elderly people in the JWs and they're also not likely to be accepted as citizens due to their becoming a burden to the nation.
Canada might be a similar country in that respect. I heard that it's enormously difficult for anyone to immigrate there if they're over 40. Canadian officials also skew their immigrants based on whether their professionals or businessmen.