AD, RJMAZ explains it quite well here.
The F-18E/F is the first airframe, the F-35 the second, and ahead of the 10-year mark for F-35 there would be another tender for the F-18E/F replacement to be chosen, etc.
It would be something like having half a fleet of Hurricanes, and half Spitfires. You then replace Hurricanes with Mustangs.
Then your Spitfires with Meteors.
Then your Mustangs with Javelins,
Then your Meteors with Starfighters,
Then your Hunters with Phantoms,
...etc.
As the RAAF at the moment has a set of F-18A's and soon to have F-18E/F. There is no need at the moment for us to purchase another set of airframes. Between the F-18 upgrade and the F-18E/F's we'll be getting, they will cover our needs and will tide us over until after the F-35 is bought online, assuming that the project (a) doesn't run overtime and (b) the cost doesn't blow out (which is a pretty big ask, I know). Then the F-35 rotates to the front, we retire the extraneous F-18A's, and that means the next replacement cycle will kick off the F-18E/F and we get a new airframe which becomes the frontline, with the F-35's rotating to the secondary airframe. Then, it is replaced with the next airframe which rotates to the front....
The theory is that your capability is re-benchmarked at each cycle with the more recent airframe, instead of having an entire Air Force of what is possibly a 20-year old airframe, looking desperately for a replacement.
The little bit of fat you have by having half of your available aircraft being a current and up-to-date-ish model allows you to potentially put off replacing the 'secondary' airframe for a couple of years if there is an aircraft nearing the end of development instead of being desperate and having to buy what is available at the time, or also is makes retiring aircraft which are at the end of service life a much easier proposition if you are only buying up half your total combat airforce in replacement airframes at a time.
I understand the theory, I'm just not sure that operating the F/A-18F in the 2020-2030 timeframe is a good idea for RAAF, and no OTHER aircraft is likely to be operated by 2020 by the RAAF besides the F-35A.
The F-35A/C is the only aircraft I consider suitable as a single air combat aircraft type for Australia. Any other aircraft, even F-22 has various weaknesses that need to be covered by other aircraft.
As I repeat from earlier, if the F-22 could be purchased by Australia, (ALL the limiting factors taken into account, not the "starry eyed" ideals of APA and their ilk) and perhaps with the upcoming "air power review" then the "rolling fleet" idea would be a great idea, starting of course with the F-22 and preferrably the F/A-18F, to roll over to a mature F-35 dominated system starting in the 2017/18 timeframe.
I'm just not sure that it's a particularly good idea with our present (and I include F/A-18F in this) combat aircraft fleet.
My preferrences would therefore be:
A) F-22 and additional F/A-18F to be ordered now and the legacy Hornets replaced ASAP.
An F-35A/C purchase to provide the bulk of our air combat force with IOC in the 2018-2020 range, replacing the F/A-18F in the current scheduled timeframe.
B) RAAF's current plan, IF F-22 can't be acquired. The possibility of a UCAV system to replace the Rhino's exists, but an enormous amount of work would need to be done for an operational capability to exist by 2020 and I can't see RAAF leading the way with such an advanced capability.
I would (almost) bet my house that RAAF would not order a UCAV until one is in-service and operationally proven within USAF/USN.
The reason for this is the capability of the F-35, which promises to be simply astonishing, if the comments of the test pilots and it's sheer statistics are even close to the truth. I would rather the single type fleet of F-35A's than a dual force of anything less capable than it.
Risks can always be managed and when was the last time a modern Western fighter force was grounded for a significant length of time? Even the F-15 fleet was only grounded for about a week and a half, max and if a war suddenly erupted, they would have been flown anyway.
Given the number of Countries that are relying o the F-35, I can't see any "grounding" issues being significant enough to compromise our security for any length of time.
It has been stated here before that the APG-65 was replaced by the APG-73 primarily because its capabilities were compromised. The USN was not going to operate a radar in their frontline fighters that possible opponents knew back to front, so the -73 was developed. The radar itself still uses the old APG-65 transmitter and power supply even today. Replacing the radar was a minor and relatively easy part of HUG. I don't know the actual cost of the replacement but I suspect you may have been able to buy one or two Super Hornets for the same amount. 
