No, not a tunnel.
Its a "long" walk from the T1 dom sec zone, out into the public area, (via upstairs/downstairs (dep/arr), past T2 & T3 and then T4).
===
I'm not sure if that's the case. There's a major roadway (General Holmes Drive) going right underneath the main runway already. I'd guess that's a bigger risk than a pedestrian tunnel.
Cost would be a bigger factor.
General Holmes Dr is cars.
But pedestrian wise, too much of a risk.
I remember years ago, MEL airport used to have that open deck area, via the set of staircase, it was a nice area, to see airport ops without a car.
Then they closed it off "due to development works" for a while, then they closed it off permanently.
PER airport has an open area, but its not open to the runway.
SYD I believe years ago, also had an open area on the top of the terminal, to view the runway, etc, but also that one got closed off.
Here in Aust, they don't let you near an aircraft, unless you have a need to, AVID/ASIC to work, or after going through dom/int sec screening if having a need to fly a service.
At least we are not as "bad" as the US, where you need a BP to get past dom sec screening, and have to contend with the TSA.
Not to mention photo id with the * on them, the state driving licence, that is.