The Australian N scale market is just not big enough to commercially justify custom made locomotive mechanisms due to the limited number of units that would be made so Australian N scale locomotives normally use chassis and motors from commercially available locomotives, mostly American locos. So it is a matter of finding a locomotive on the market that is a close enough match for an Australian loco or one that can be relatively easily modified to match. This obviously makes Australian locos more expensive because you usually have to buy an American one, buy the shell for the Australian loco, then strip the outer shell off the motor/chasis an fit the Australian shell.
Some Australian locomotives just don't have a commercially available locomotive with close enough match. An experienced, dedicated modeller might go to great lengths to alter a chassis to complete a treasured model. I don't know about you, but that certainly rules me out.
For several years, LifeLike SD7s where used for their gizzards as these are a reasonable match for quite a few locos, including Victorian B and S class locos. The structure of these also makes them relatively easy to adapt to longer or shorter wheel bases by cutting a piece out or inserting a piece into the chassis and replacing the plastic tubing that is used for the drive shaft with a longer or shorter piece. This was the case for early G class locomotives needing a longer wheel base. This is a fair bit of fiddling and you have to strengthen the chassis now that it has been cut. Later G class kits came with a casting for a replacement chassis, meaning you have to take all the working parts out of the original chassis and put into the replacement chassis. And of course, you still need to find some plastic tubing of the correct diameter to extend the drive shaft. I have done one of these (Model G514 on this site) and it was certainly testing my skill level and I wouldn't want to do it again.. I wouldn't call it my best running loco, but it does run and gave me a G class loco to put on grain or container trains. Luckily, G Class locos are now available Ready-To-Run, using different mechanisms so there's no need to do this anymore.
B class and S class locomotive kits currently available from Pete Boorman's Workshop are designed to fit LifeLike SD7s. In their original form, while a good match in length, SD7s are too high to fit inside most bodies used in Australian models. Rotating the motor so it sits sideways lowers them enough to fit, though this will only work for locos that have full width bodies such a S and B class and not those with external walkways such as X class. Refer to the note on SD7s for more information about these alterations. The other catch to using LifeLike SD7s is that few Australian shops stock them so it was common practice to get them mail ordered from America.
Nowadays, Atlas SD7s have become more popular due to their superior running quality. I don't know how much effort is needed to adapt the Pete Boorman models to these mechanisms. I'm still using LifeLike mechs for these, they may not be as good but they are adequate and good value for money. I have heard that LifeLike is now making SD7s with split chassis style mechanisms similar to the Atlas SD7. I haven't seen any of these so cannot comment on their suitability.
Although they have different exteriors, Atlas SD9s, SD24s and SD26s use pretty much the same mechanism as the SD7 so these can also be used. Some have more intricate work in their exterior making them a bit more expensive. Since we just discard the exterior, get the cheapest one you can find. SD35's are fairly similar but a little bit taller making them unsuitable for most Australian models.
One feature of the Atlas SD7s is that the bogies are mounted slightly forward of centre. They can be reasonably easily rotated thereby extending the wheel base. Whilst this may still not be perfect, it is generally considered good enough, given the lack of viable alternatives. This is what is done for the new Ready-to-Run G Class locomotives. See model G512 on this site. G and BL loco kits released by SAR Detailers are based on the same master so use this same strategy. Freight Australia G513 on this site is an example.
Atlas VO1000 are an excellent match for series 1 "Flat Top" T Class locos. The Ready-to-run model currently available was designed to slip straight onto these without modification.