What should I wear to attend this dinner party?

I’m travelling a few hours out of Sydney by train to somewhere else in NSW over the weekend, to attend a party. I’m staying overnight at my girl friend’s house (just a friend). It’ll be a dinner party at a fancy USA type bbq diner. What should I wear to the party and what kind of change of clothes should I wear for the next day, to travel home?

What should I wear to attend this dinner party?
Post Opinion