Aussies passports are some of the most expensive in the world. I had to renew mine awhile back and I had two pages (and a half) left unused in it at the time. I thought I would use them up before the new one came in.... but I didn't - and at $20 a page that's a lot of wastage.
Hack 1: When you renew a passport at the Aussie High Comm they give you six months to collect it. The clock is running though already on the expiry date - so do you try to use up the last two pages while the clock is ticking? I went middle road and waited a month and managed to up use an entire page thanks to Laos visa stickers. At this point I made an appt and went in to collect mine. Previously when I did this the staff at the AHC (who frankly, are usually wankers) would either cut the bio data off or mark the PP as cancelled. This time, neither - passport was returned unmolested by a nice.... who did say "The old one is cancelled (how? Via interpol? Only into Australia was my bet) this led to ;
Hack 2: I updated my REP details to the new PP but I also had a trip coming up to a place in MY I KNEW would stamp the crap out my nice new PP. So I registered both the old and new ones for a MY digital arrival card. When I showed up in MY I acted like a befuddled old man and passed across the OLD passport. The nice lady actually went carefully through it and found an unused space (not a page - a space!) and commented she knew I was running out of pages and didn't want to waste one. After bending her over the desk and rogering her I was left wondering... "While I get out of Malaysia?".
Well I did, on my old "cancelled" Aussie passport. Do I care what the Aussie wankers at the High Comm think who are reading this? NOT AT ALL. Your mileage may vary. Technically the Aussie govt (aka Prison Admin) allow dual passports but it's really a pro Israeli thing (you basically need to show that you're travelling to Israel and the Gulf Nations a lot) and they rarely approve. I suspect I would have trouble using the old passport to get into some nations (and probably back to Australia even though the passport is still unexpired it would flag up in the Airline pre approval system).