Before you all get too excited about "Allagan ruins" in Qarn, the main architecture we see there is Belah'dian from the Sixth Astral Era. It's only some of the deeper parts of the structure that appear to be older.
Also, this happens to tie into an interview I got linked to the other day, which goes into the link between the Meracydians, Allag and Qarn - though entirely disproves the "Qarn is Allagan" idea. At least the familiar part of it.
To summarise the explanation:
1. Allagans discover the Cathedral in Meracydia, recognise "Azeyma's symbol" being used by worshippers of Sephirot and conclude that the symbol has shared origins somewhere in the past.
2. The Cathedral is transported to Azys Lla and its architecture is studied.
3. Popular knowledge of this study leads to the designs being incorporated into modern buildings for aesthetic purposes.
4. The Calamity happens; Allag's knowledge is lost, some of its buildings survive.
5. Later peoples mistake the decorative features on these buildings as "arcane wards" and again note the apparent symbol of Azeyma, the Warden.
6. These designs get incorporated into their own architecture, including Qarn.
To quote: "It also helped that Qarn was built atop the remnants of something much, much older than the Belah'dian civilization".
Also, from the lorebook: the ruins in the Sagolii Desert are Allagan, while Qarn and the Invisible City's architecture (also found in the Footfalls in western Thanalan) is definitely Sixth Astral Era Belah'dian.
So it's possible that the older base under Qarn is Allagan, or perhaps older again, but Qarn itself is not. I would guess it's older than Allag, or they'd be relatively likely to mention it rather than being vague.
Funnily enough I was in Qarn Hard the other day and noticed that the sun sigil on that puzzle looks suddenly significant... well, so much as a pre-established astrological symbol can be claimed, anyway. But I think it's glowing in the same colour as Azem's soul crystal.
Qarn Hard is generally odd though, because all these extra traps and defences have just popped into existence in the same actual spaces we already traversed.
Now which one out of Hades and Hythlodaeus would be the moon symbol, and which one the star? I'm sure we can wrangle some kind of situation for a certain nation-building Ascian to be involved in building this thing.
The minaret-type ruins are definitely Allagan according to the lorebook (and probably the sightseeing log), but they don't resemble the Qarn architecture at all and as I mentioned above, are confirmed to be from the Sixth Astral Era - somewhere around a thousand years ago VS Allag's five thousand.
In-universe the minaret style seems to be maybe from the same era as the Crystal Tower, and drawn on for the Crystarium's design.
The carved statue golems seem unique to Qarn, but the rocky golems in general (which are also found around and within the temple) are actually pretty strongly linked to Allag. They appear around and in the Binding Coil and in the trial of Bardam's Mettle, which is full of Allagan mechanisms.
(...actually, Qarn and Amdapor. But they were on the other side of the War of the Magi. And Nym, which was the other other side of the War... but anyway. Allag.)
We've still seen no proof that anything distinctly Australian is from Meracydia, while what little we have - primarily the plants aloe and sesame - points more to Africa.
If anything, the lack of identified origin (to my awareness) for the koala and frill-necked lizard minions is a sign against them trying to link Meracydia to Australia.