So after a frustrating couple of weeks waiting on one ebay seller to get back to me, turns out the delay is all due to Toll IPEC receiving my item and then deciding to lose it for a week. They still haven't found it. Ironically their marketing slogan is "Toll Ipec - If It's Urgent", which should then be followed by "...don't use us, because we'll lose your items and not let you know about it". Pretty impressive for a courier company to fail so hard at their job, so in future I'll be making sure to avoid using them whenever possible.
Anyway, had an excellent experience with Better Hardware. I placed an order Sunday night with a note I needed these screws urgently - I got a call Monday morning from their sales manager who offered to meet me in Melbourne and hand them over to me! I said the next day would be fine, and he couriered them to me which I received Tuesday morning today! Rare to get such great personalised service, highly recommended. So if you need any decking supplies, email Tim at BetterHardware directly and say you saw this recommendation on Tim&Tina's blog and he'll look after you!
I also made a jig out of some leftover aluminium fastener plate - basically I wanted to screw each board down one at a time and make sure all the screws lined up correctly. I staggered the screw joins too to try and minimise any crack formation in the joists.
So, long story short - lots of measuring, re-measuring and re-re-measuring, I manage to line up the first decking board and the first two picture frame boards, and lay a few more decking boards down before my cordless drill battery had enough.