Thanks for the info Nux. I was pretty sure this was the case but the wording on the Qantas Lounges page made me rethink it.
"Next onward flight that day must be a Qantas marketed flight (QF flight number).

One guest allowed and must be travelling with the member on the same flight."

Not to worry though. I did really want to check out the F lounge as I have never been there. But I know the J lounge is more than sufficient for a pleasant experience so I will just have to use a lounge pass for this one.