I went to Australia last year.
I have gone to Australia in the past year.
I know it is perfectly alright to use past simple for the first sentence.
But why do you use present perfect "have gone" for the second one ?
I went to Australia last year.
I have gone to Australia in the past year.
I know it is perfectly alright to use past simple for the first sentence.
But why do you use present perfect "have gone" for the second one ?
Because the present perfect refers to past time with present relevance, it is not usually used with a specific time reference that does not include the present (such as last year), but it is usually used with a specific time reference that does include the present (such as in the past year).
If you said I went to Australia in the past year it would imply that for some reason you were regarding the past year as a time not stretching up to the present, which is a bit strange.
WS2 brings up a further complication, which is that in, perfect constructions only, have been to means went to and left again, but have gone to usually means went to and stayed. That is an idiosyncrasy of the particular verbs be and go in the perfect.
I think in 2 you perhaps mean I have been to Australia in the past year.
The perfect have/has gone is legitimate, but not in this context, as it suggests the person is still there e.g. He has gone to Australia on business. One wouldn't say I have gone - since one would be there, and would need to say I have come to Australia to watch the cricket.
It is sometimes possible on one's return to describe multiple visits I have gone to Australia three times in the last few years.
Just to focus on the latter parts of your examples:
The first one implies the previous year. As this year is 2015 "last year" implies the period from the first of January 2014 to the 31st of December 2014. The second one implies a time period starting 12 months prior to the current date (so 18th October 2014) to the present date (18th October 2015 at the time of writing this).
Ergo it is possible, if you went to Australia in May 2014, that you went to Australia last year but you haven't been to Australia in the past year.