"Typically, Australian banks pay a small premium to swap foreign currency into Australian dollars. This premium is also referred to as the basis, which is the difference between the implied cost of obtaining Australian dollars in the FX swap market and the cost of obtaining Australian dollars onshore."
I understand, Australian banks issue USD bonds to finance their AUD assets. They then use FX swaps to convert USD into AUD. The author mentions they pay a premium which is the FX swap implied rate less OIS AUD rate. Given this premium is usually positive it means FX swap implied rate > AUD OIS rate. So what is the reason why banks pay a higher rate in the FX swap market instead of using cheaper onshore funding (AUD OIS)?