ok i got this...
best way to do it...
go see a lawyer. give them a sitrep and they'll tell you what you can and cannot do to protect yourself. if all goes well, they'll make a piece of paper that says so-and-so is a roommate only, paying for rent and providing for themselves only. worst case, you basically walk out with a prenuptial agreement of sorts. may set you back a few hundred but beats a document you wrote up at home in comic sans.
your gf seems like a reasonable person... she'll have to sign it so may as well talk it over with her.
and if you havent got a will sorted, might as well do it while you're there.
( i recently learned that beneficiaries of your super is not considered part of the will, but an insurance payout.)