one question.

Goverments (except Canada-Usa) shares informations about the person's PAST immigration status to the other goverments

Because i went to the Canada on a student visa (2002) and after 5 months i applied for refugee status , but they rejected my case and i came back to my country -voluntary-

i know that voluntary departure looks in the records ''departed on time''
so i am fine with that. i also received police clearance says that, i have no crime record etc. thats also good news for me.

now i am gonna apply for a spouse visa and my lawyer said not to declare it because '' privacy act'' prohibits access to such things from other goverments. unless you apply for a another refugee status in another country.he said sometimes they might share fingerprints for crosscheck.

so i am really confused , i dont know what should i do

I doubt that you'll get too many people on a forum actually too knowledgeable on just what is shared between what countries.

A Golden Rule in my book as far as Immigration goes is always to be honest.

I would expect that if Immi do find out of past matters not declared that should have been, that would not be in your favour and a refugee application/rejection and leaving a country may affect a partner visa application less.
A lawyer probably has less to lose than yourself.

i read somewhere that britain, oz and canada shares immigiration database.

yes they share..

they only share if you apply for a refugee status with no document to be able find your identity using your fingerprints.if they find a match then they share other details.