$60K AUD per month is dirt cheap. In 1990 a 737 cost $300,000/ month. Interest rates on commercial leases were (I think) 11%
Figure out if you'd be happy to get that kind of return on a $100 million asset. (I know, I know...they’re old aircraft but since 737s are only really good for about 20 years, you really would need about $7 million/yr average to break even. (Back of the envelope calcs)
I wonder if they are paying for engine time separately?