It would have been nice to go to Australlia this winter, but there is no way we can do it. (M. Swan, "Practical English Usage", Topic 259.3)
You're asking: "How can we determine what kind of unrealised event (past, present or future) we're talking about?"
The use of "would have" + past participle tells us that there was some possibility in the past but it has not been realised.
Let's say it's October 2014 at this moment. In March 2014, you were earing $4000 a month and were full of hopes to travel to Australia in November 2014. But in May 2014 you fell ill and had to leave your work, returning only in September 2014.
So, at the present moment of 19 October 2014 things have already turned out so that a trip to Australia is out of the question. By using would have been, you signal to the person you're talking to that some unspecified past circumstances make this trip impossible. It could be not some single event like sudden illness but the whole development of your life prior to this moment that makes this future trip impossible.
How do we know the impossible trip is positioned in the future? Because the independent clause "but there's no way we can do it" is in the present tense: we cannot do it now. It is not necessary to harmonize an independent clause with another independent clause.
To place the timeframe of the event in the past, we may tweak the independent "but-clause" to the use of the Past Simple:
It would have been nice to go to Australlia this winter, but there was no way for us to do it.
Now, we say that there'll be no possibility to make the desired trip even if we get a payrise and a paid leave right away, because, for example, the season has passed (it's 1 March 2015).
I'v found a great passage by a native speaker on sentences of this type:
I think the best way to think of what is going on is that this kind of sentence describes a 'lost opportunity'. And in describing this kind of 'lost opportunity' English only allows past conditionals, even if that fact has consequences for today or for the future. You can think of it as "the opportunity is over and has moved into the past."