I think the 6 Lion Air Australia planes will be newly delivered from the production line.
If Lion Air Australia wants to have the planes maintained in Asia, it will have to be at a facility that has been approved by CASA. If Lion wants to operate the planes on EROPS, the facility will need that approval as well. Besides, it is a long distance to ferry Australian based 737s somewhere in Asia for periodical checks, when there are lots of Australian or NZ bases within close range of SE Australia.
Talking about range - the range of a 737-900ER is about 3200 miles, right? Indonesia is the only Asian country within that sort of range from SE Australia. Bali is well within that range. Even Jakarta is just on the edge of the published range of a 739. I do not think there is anywhere else in Asia you could fly to non stop from SE Australia in a 739. Singapore, Malaysia look like they are beyond the non stop range. If you have to do a stop in Darwin or Cairns on the way to Asia that seems to limit what the plane would do for you. As a plane for Transcon Australian routes - Brisbane-Perth, or Trans Tasman the 739 would be a great plane.