It was pre covid but I had a long layover in Melbourne on a through ticket and they let me into the F lounge. I didn't think it would be an issue. They had a little chat between themselves then let me in. I have a feeling it was only 4 hrs (possibly 6) though. It definitely wasn't 8hrs. A 6hr layover I'm generally getting out of the airport, or even 5 hrs.

I'm certain there are luggage storage places at Sydney airport to drop hand luggage. I've also used one at Central station.