Sydney to Samui is always quite expensive, pre covid the cheapest was to fly Sydney to Surat Thani via KL, and then get the shuttle to Nathon for 400 baht. I don't think AirAsia resumed the KL to Surat Thani flight.
In other news seems that Thailand pass is getting abolished in July, so Samui may get a bit of high season taste. But some venues in Chaweng like Hard Rock cafe and Palms are still not open, and the Ark Bar renovation seems to go slowly slowly...I doubt they'll be able to open with its former glory early August.