Outstanding invoices from customers - when should i pay gst?

Hi folks, I have a quick question. 

My business works on a client servicing model and we raise invoices after the service has been provided, and clients usully pay me in 1 to 3 months. Unfortunately some clients make undue delays and sometimes they get pending till over 6 months (even more in rare cases - usually government companies take a very long time to pay ). Sometimes it becomes a legal matter as in the rarest of rare cases they try to say they have become bankrupt etc and avoid paying the invoices.   

Under GST, when should I remit the GST for these invoices? When the customer pays me, or as soon as the invoice is created ? 

Some people have told me that the GST should be paid for each months invoices, even if the customer does not pay me.

If I have to remit GST even without the clients paying me, I would have to just wind up  :( So kindly requiring clarity on this matter

Thanks in advance.