How to pronounce long consonants in Polish?

I’ve always thought it’s quite simple, natural and straightforward (just like devoicing). In words like Anna, you just pronounce a long (strong) nn: Anna, not “An-na” (like you were stuttering). But recently, I lost confidence.

I know that such words are rare in Polish but I’m still interested. Do you pronounce them like Italians (for instance, Ferrari, Giovanni, fabbrica, Pippo, macchina, mezzanotte, cappuccino) or you “separate” them or you shorten them...?

In speech, it is normal not to leave any kind of vocal pause between the words in a phrase or sentence.

I want a cup of tea. < no gaps, so it's like Iwantacupoftea.

If you have the same letter at the end of one word as at the beginning of the next, you pronounce both of them.

Bus stop
I'm mutating
Fish shop

You will probably find you've effectively doubled the lengths of those consonant sounds.

Now try a popular Polish word: inny.

Surely it's as simple as just making that n sound for double the length of time than you would if there was only one n. Maybe this lengthening in Polish is slightly more accentuated than the English examples I gave, but I'm sure there are no other odd things you need to know about it - it doesn't affect the other sounds around it in the word.

I've just remembered a good example of a double-length n in English:
Stubbornness.

Theoretically we don't have long consonants in Polish. When we have two identical consonants next to each other, we pronounce them 'separately': lek-ki; mięk-ki; in-ny; man-na. I mean, ideally, i.e. in theatre, speeches etc, when we care to speak very clearly and correctly. The first of the pair is pronounced a bit weaker but one can hear these are two sounds.

But in natural speech some of the double consonants are pronounced like one long consonants (those, that can be pronounced longer, so no plosives or affricates) because it's easier. So, you can hear manna, Anna, panna, hossa, bessa etc. with something that is actually a double-length consonant.

You can even hear people pronounce one consonant instead of two (e. dżownica instead of dżdżownica) but that is not correct. Some people, especially children, say letki instead of lekki but that is incorrect, too.

which would be the correct way to pronounce two identical consonants on the end of a word eg ... jestem z Australii? One silent or both long?

'i' is not a consonant.

Australii is pronounce more or less like australi.

of course it's not, ;) I knew it was a vowel but was more interested in the pronunciation of double i's at end of words. So one i is silent, many thanks for explaining that.