After searching for over an hour for a 5m displayport 1.4 cord available to Austalia, I thought I would post it here in case any else is searching.
This cord is listed on the certified list on the displayport website.
I haven't tested this cord yet - the longer cord length has a chance of resulting in some signal degradation.
normally i'd say BS for expensive cables but can confirm that the certified list is important for longer cables. I tried a number of cheapo 3m displayport cables and had all kinds of trouble until dropping ~$50 on a certified cable from the same list. had zero problems since then