I'm travelling soon to Auckland going to Manila and has a layover way back in Sydney for 12 hrs. Can I leave the airport? Or do I need a transit visa or whatsoever? I'm a first time traveller with NZ passport holder.
I'm a Kiwi too, and I assure you, you're totally fine to leave the airport.
Evidence? Just 6 weeks ago I was flying through Sydney enroute back to NZ, and had 10 or so hours to kill. I caught the train into town, met up with a couple of friends for lunch and wandered around a bit, and then headed back to the airport.
I didn't even need to collect my luggage - it was checked right through, so when I checked in for the next flight, I just confirmed that with them. No problems whatsoever.
If you have the new NZ black passports, you can even use the Smart Gates - where you just walk through with a photo taken of you and a scan of your passport, don't even need to queue for the customs. It's all set up to be very straightforward for Kiwis and Aussies to visit and / or work in each others' countries.
Enjoy Sydney. It's a bit expensive to get into downtown, but check out the Wikitravel page about how to get around that if you want to, otherwise take the train - it's pricey, but it's so easy to get in and out of the city.