Jellicoe actually recommended either Sydney Harbour or Garden Island in WA as the prefered location of the RN base that ended up being built, at horrendous expense in Singapore, what if he had been listened to and Sydney was developed as the base instead?
The obvious difference I can see is the Japanese would not have been able to put the base out of action, let alone, capture and use it against the allies. With no major base to defend, hopefully much of the men and material lost in the defence of Malaya and Singapore, including Prince of Wales and Repulse, would actually been deployed in the defence of Australia instead. The existence of a powerful squadron in Australia, rapidly grown to a fleet, as well as a large, intact, standing army, means Australia (and the UK) never need to kowtow to that deeply flawed, ego maniac, MacArthur. This actually leads to a much more effective SEA and Pacific campaign as the British Commonwealth, including Free French and Dutch forces, take a much larger, more visible role.
The key to this would be the money invested in Singapore is instead spent expanding and improving facilities in Sydney and forward bases in Darwin and Perth (to cover the Indian Ocean). This would include BB sized dry docks, repair and replenishment facilities, as well as Naval Air Stations, defences and garrisons in all three locations and considerably expanded and improved shipbuilding infrastructure in Sydney in particular.
Also following Jellicoe's recommendations Australia would have greatly expanded it's coastal and continental defence capabilities, including small cruisers, sloops, submarines and above all, aircraft. Local construction of aircraft would have been a priority. Ideally with Hawkers setting up in Australia to manufacture Hawker and Gloster designs and CAC manufacturing aero engines, license producing aircraft, including trainers, general purpose aircraft, patrol, medium and heavy bombers, flying boats and amphibians, as well as aircraft of their own design. De Havilland would mainly produce civilian and commercial types but would begin building military types as war approached, starting with Tigermoths and militarized Flamingos
![Grin ;D](http://beyondthesprues.com/Forum/Smileys/clabIcons/grin.gif)
and ending the war building Mosquitoes, Vampires, Hornets and evolved Flamingos. DAP / GAF would never have existed.
An improved rail system would have been a necessity, linking Sydney, Perth and Darwin with a standard gauge rail. This would have required significantly expanded railway workshops and locomotive and rolling stock building and maintenance. This, apart from significantly improved communications and logistics, would have provided for a massively increased potential to build armoured vehicles.
In addition, the decision to construct the major British base east of Suez in Australia, gave Harry Chauvel got his way, mostly, and was able to establish a motorised and mechanized, standing army. It was not as large as he desired but it did exist and it was fully mounted with organic trucks, cars and armoured vehicles. This standing Army would have been based on the AIF with the 1st Division and 1st Light Horse Brigade being made permanent and most of the remaining formations becoming the new reserve force, while the Militia remained as a geographically organized as a training cadre and local defence force. Because Australia was now a major British base it became necessary to remove the restrictions on the deployment of Australian military forces outside of Australian territories. Also the presence of British forces, RN, army and RAF, their bases and training facilities, was a further boost to the professionalism of Australian forces and the Australian economy, in fact due to the high esteem Australian forces were held in following WWI, the British garrison was smaller than it would have been otherwise with the UK funding additional Australian units to fill many of the intended deployments instead.
These bases and the flow on effects would have seen massive expansion and modernization of the Australian economy that would have then expanded exponentially with the approach and onset of WWII. Australia would have become a major producer and exporter of military material, supporting the allied war effort and generating an even more massive lend lease credit, establishing Australias place in the world for decades to come.