Check the PO Box address for each company- not uncommon to put Capital Assets in an entity other than the trading company for all sorts of reasons- in my experience. 
Beyond the postal address- given it is not a "known major" leasing company I'd say its a fair bet that they are owned- which of course does not preclude them being leased from one into the other, pending how the relationship is structured- not that that is anyone's concern!!!!!! 
Registration holder as of 19 February 2008
OSNET JETS PTY LTD
PO Box 136
BELMONT WA 6984
AUSTRALIA
Registered operator as of 19 February 2008
NETWORK AVIATION PTY LTD
PO Box 136
BELMONT WA 6984
AUSTRALIA