There was a promotion specifically for Australia to Hong Kong - I’m not sure what the fare difference will be. I can check but don’t think to would work. I also may just use Asia Miles for the Hanoi leg - just waiting for them to release it - as they seem to have high J fares for that route.

Yield management software (for revenue and award) know full well when CNY and Tet are and the expected load on the flights.
Don't expect bargains if you travel near those dates.
Linking the PNRs won't achieve much. It is just a note in the PNRs.

I know. It’s why I locked in my fare early from SYD. I suspect they’ll release availability for HAN any day now…

I don’t know why you suggest I’m looking for bargains.

That's not what I suggested -merging PNRs would work if CX will do it for you.

I miss real Travel Agents for whom these things were trivial.

I dont think that a merge is an actual merge once you already booked separate tickets.
It is just note in the s2 PNRS.
Some TAs can sell you a ticket by stacking together separate tickets. The outcome and risks can be unpleasant. The best approach is to try to change th first ticket to a SYD-HAN. That could be impossible om a promotion fare , or vey costly, or less given that SYD-HAN are often Cheaper that SYd-HKG.
But ticket needs to be changeabe or refundable. Despite of the change/refund fees, and the high fares in CNY/TET busy holiday season.
That would be my first action to get full protection.

Second option would be to buy a secondH KG-HAN ticket as you planed and hope for the best.
You have to compare the two costse, risks and decide accordingly

I think brunos and I have the same thought. Ignore the misconnection risk and just fix the visa risk in advance. It sounds like you're used to this and also got a good deal on the original fare (maybe? I'm still not convinced but you can look in a few weeks and see if that's the case. It's rare that a direct flight into hong kong is cheaper, but I mentioned CNY and with Tet you've the same challenge, though for a single country).

Indeed, we are in agreement.
Like you said, I would have waited till fares SYD-HAN had been made available. Whatever promotion you got on SYD-HKG (including a lot of restrictions),it is likely that you will end up with a larger cost to book separately the HKG-HAN. Anyway HKG-HAN is a very short flight (flight time less than 2h0, so Y is quite bearable.

CNY and Tet always fall on the same dates (2026 17 February), so it is an extremely busy travel season in Asia.

Is syd-han during February a market that commonly sells out a year in advance? What's the source of that demand?

Tet

I flew from NNG to HAN and back during CNY (same days as Tet) in 2019. While I know the markets aren't not apples to apples, that ticket was dirt cheap one weak out and the flights were 75% full, at best.

Well yes, but the Vietnamese population of New South Wales is rather bigger than the one in Nanning!

I believe the CX check-in counter at SYD can also print the HKG-HAN boarding pass.
It is not possible to merge 2 PNRs after these PNRs have already been created, period. TAs cannot do that, either. This is nonsense.

While TAs can sell separate tickets within the same PNR, it is not possible to do that on a CX-CX connection. As long as the 2 flights are within 24 hours, they become married segments. On Amadeus, ticket cannot be issued on only some but not all of the married segments. On Sabre, this is technically possible, but once CX detects it, the booking will be cancelled, and the TA will be expected to pay a fine.

Well I guess things have changed. My TA used to do this sort of thing regularly back in my road warrior days. Perhaps not "merging" the 2 PNRs as such, but replacing them with a single newly booked PNR against the same tickets.

With paper tickets, assuming availability exists (not a problem back in the day if you were GO or DM, given seat guarantee, or TA can ask CX to release seats), yes you can create a new PNR and re-associate the old tickets in the new PNR. This is technically also possible with e-tickets, if you have fully flexible ones, but much more difficult in execution (still, possible, so long as the flights are not within 24 hours and not married, or if the flights are marketed by different airlines). Alternatively, TAs can book "Ghost Confirm (GK)" segments in a PNR to present a single itinerary for you, but these segments are "passive segments" in the sense that, in the airline's reservation system, the 2 PNRs remain 2 PNRs. A single PNR is just created to make it easier for you and the TA to see things. At the airport, they will still see 2 separate PNRs.

I don’t disagree with you - I’m just hoping and suspecting they will release some reward seats on the flight I want. The sale was going to end before they released onward connections to HAN for some reason.

Unfortunately there are few very good options to get to HAN from SYD as there are few direct options - and ones I would consider paying J for - and the price was good via HKG.

Re the booking in advance - we found Tet did book out early this year - so I did want to book early.

Finally, I do appreciate everyone that has provided advice. It has been really informative and I thank you for sharing your knowledge.