Is that for a sustained period of time or are you referring to a pandemic-related anomaly? My impression is that the [East Coast]-PER-LHR flights in 2018/2019 were consistently more expensive than almost every other airline - I wouldn't ascribe much importance to one-offs here and there as travel is restarting. Right, prices seem middle of the road but certainly not cheap. For comparison, the cheapest SYD-LHR return available for random dates in July is on Sri Lankan at $2000 followed by Thai and Korean Air at $3000, then Qantas at $3900 via PER (with very similar prices on Emirates, Qatar, and Etihad, and Singapore Airlines coming in at $4500.)
