It’s the same with Sydney
eg. 88.9 89.7, 90.5, 99.3 and 100.1
Not sure about a story behind it as such, just the government decided that multiple low powered suburban community stations would add more diversity than a just few high powered ones in their place.
Because they are low powered, nearby stations can be placed on adjacent frequencies without appearing to cause issues,
Through careful planning, you could easily condense the sub-metro community stations below 95 MHz - I once mocked this up, wonder if I have the file somewhere
Good luck trying to find reason in ACMA frequency allocations especially around metro areas. 
I would say that part of it was the early use of the FM band being for TV transmission. So the high powers were preferably placed at the higher frequencies (Channel 5) and low power FM were allocated frequencies that would not interfere with TV channels. From memory the adjacent markets to Melbourne would have been ABNT3 and ABLV4.
I dare say there’s an element of truth in this. I’m pretty sure the reason why 91.5 was never issued for FM radio in Melbourne related to its potential to interfere with the audio of ABNT3. Though I guess this can’t explain why EON FM was given 92.3 in the early days
It’d be a huge exercise, but there would be a good argument for a full restack of FM radio frequencies, particularly around the congested metro areas near Sydney, Brisbane and Melbourne. It would eliminate some of the awful doubling-up of frequencies that causes problems whenever summer comes around
Brisbane sub metro community stations are spaced 99.7, 100.3, 101.1.
The latter two are adjacent markets, easily heard in each.
There’s also 101.5 Caboolture with its site out Woodford way, long way from Logan on 101.1. Pre Nambour FM, it used to be on 91.5.
100+ km away, Beaudesert is on 101.5 too.
Further out again, Boonah is on 100.1, no problem with 100.3 out there either in high spots where they can both be heard.
Here’s the SA ones:
Yorke Peninsula & beyond:
Correct, DMG actually had the winning auction bid for 91.5 the first time around but as the huge ERP of ABNT3 was not cleared, when it came time to ordering the antenna etc, Steve Adler and co reminded ACMA who switched it to 100.3 for Nova. By the time 91.5 was auctioned, again to DMG, ABNT3 had been cleared.
which worked well for all concerned. Nova was right next to Mix and Fox and The Pulse which had 100.3 got the high powered 94.7.
I’d agree there would be a good case to replan the FMs in the eastern capitals. South East Qld in particular is a mess of overlapping centres from Noosa to the NSW border. IMHO they also need to find space for another commercial FM licence for Brisbane and Perth. Brisbane has almost 2.5 million people and only 4 commercial FM stations. Perth is approaching 2 million people. These two markets are under-served. At least Brisbane has some good overspill from River 94.9. There’s plenty of room just bad planning - e.g. it’s absurd a multicultural talk station like 4EB is on a high powered FM channel 98.1
I’m pretty sure that ABNT3 was offset by a full MHz to 92.75 MHz for the mono audio signal (based on reception here in Brisbane on occasion).
Caboolture and Beaudesert on 101.5 clash horribly in the middle. I presume they’re probably both ok in each other’s respective licence areas, but even as far south as the Logan Mwy you still hear Caboolture causing the more local Beaudesert signal significant grief
And why is 99.7 on the standard Brisbane spacing of channels, when Bay FM and 101.1 Logan on the Sunshine Coast spacings? It just seems a bit odd
Legacy arrangements before the implementation of more rigorous planning in the early 1990s explains the apparent haphazard placement of some stations. For example, in Coffs Harbour, 2CHY FM (104.1) dates back to the early 80s with a totally empty dial in the region; they could choose whatever frequency they wished. The cool kids running that youth station in the 80s probably chose 104.1 to grab Sydney and Brisbane listeners when they visited the Big Banana.
The other transmitters in the Coffs Harbour region use the Melbourne or Adelaide channel allocations- Peter Perfect might have wanted to move 2CHY to 103.9 or 104.3, but the status quo was preserved where possible. Bathurst is another area with channel allocations all over the place, with 2MCE-the legacy station-on 92.3, the low-powered Mt. Panorama stations on the Adelaide channels, and the high- powered stations from Mt. Ovens on the Sydney channels.
4DDB is another station on a legacy frequency of 102.7 that fits into no channel spacing - it should be on 102.9 based on the rest of the region. It could have been moved when 4ZZZ, MMM, BBB, MIX etc got new frequencies.
Nobody mention Rebel Media on 99.4 and 100.6 
102.9 was allocated to 4BH after they bid for the 3rd Brisbane licence. They were originally allocated 96.5 but asked for a frequency further up the dial towards 4MMM and B105. Of course they never came up with the money in the end and didn’t concert to FM. It would have been a nice choice in the 90s having:
106.9 QFM
105.3 B105
104.5 4MMM
102.9 4BH-FM
I thought the reason that 102.9 wasn’t available was that 4DDB would have needed to change frequency
Not sure if 4DDB had to move or not but they definitely allocated 102.9 to 4BH. They did so reluctantly and there was some talk at the time of interference. But if I remember correctly the concern was of potential interference with a TV channel at the time (perhaps TV0?). But they still allowed it.
I have wondered what format 4BH would have taken to FM, but assume it would have been easy listening or classic hits. I imagine it like 5AD-FM in the mid-90s. In the early 90s QFM had a somewhat soft AC format leaning towards newer hits at night.
Has anyone had a chance to check out RadioApp. Is it running the promised full set of commercial stations plus ABC and SBS?
I had a look at it through both my iPhone & iPad, and they do indeed have every ABC, SBS & commercial radio stations (including digital-only stations) on there, except for the ones that are not streaming (ie. C91.3 Campbelltown, 2GZ/Star FM Orange, 2BS/B-Rock Bathurst, Aussie/KOOOL Digital Melbourne & BOG/SRN stations).
I think the new RadioApp could easily rival TuneIn, at least when listening to the Australian commercial stations. You can even search stations by location, something you couldn’t easily do on TuneIn, which can come in handy if you’re focusing on just listening to stations from a certain market. Plus, unlike on TuneIn, you can easily listen to the Grant Broadcasters stations such as River 94.9, Zinc 96, Power 100 & 2ST, among many others.
Thanks - good to know. This was originally promised for late November. When it was just SCA I also found it good that the steering wheel controls could be used to change station like iHeart Radio but not Tune In.
I recall that when 102.9 was allocated to the Gold Coast for a commercial licence, that even then, interference to 4DDB was still a potential concern.
Two solutions were proposed:
a) A null be placed on 102.9 towards Toowoomba
b) 4DDB move to 102.5
I don’t think either was required in the end,
The 4CC translator at Agnes Water on 98.3 is a Barry Crocker; it relays the 927 AM signal off air! I have come across similar setups in remote mining towns, but this area is much more populated.