Low level deck - second opinion need.

I'd be using 100x100 posts.

Hi I'm assuming that when you say "all timber will be hardwood", the supports will be F15 hard wood? I was just going to mention that unless you are getting red gum (which costs a squillion), hardwood is mixed. What you end up getting is timber that has the same lifespan as treated pine, plus the hassles of installing decking on hardwood. So a lot of timber merchants I have talked to either avoid stocking hardwood and/or insist on treated pine. Either way, below you will see is AS1684 with F5/F7 specs. The supports are rightsized to AS requirements which should give you a comparison and cost. Similar to Stevoh, I would go 90x90. I would also consider 5mm gap not 3mm. Timber Requirements

Bearer - 2 x 140x45 F5 TP laminated, 60mtrs @ $5.85 Cost $351

Joist - 140x45 F5 TP at 428.6mm centres, 129.9mtrs @ $5.85 Cost $759.915

Decking - 90x19mm wide board with 5mm gap, 602mtrs (incl 10% waste) @ $4.85 Cost $2919.7

Total Timber Cost $4030.615

Screw and Support Cost

Decking Screws - 50mm 10g screws 2 per joist, 6 boxes of 500 @ $60 Cost $360

Supports - 300x90 Stirrups, 20 stirrups @ $8 Cost $160

Concrete - 0.92m3 concrete delivered, 0.92 m3 @ $340 Cost $312.48

Total Support Cost $832.48

Total Project Cost $4863.09

Support Detail

Bearers - 5 with a bearer span of 1800mm supported by 300x90 Stirrups

Joists - 15 spaced at 428.5mm centers

Concrete - 20 holes 300x650 wide and deep

Forgot to mention I was using stirrups. Calculator wouldn't let me select that option.
So apart from the stumps does it look correct?
If I'm on the right track I will know if I have got my head around the span tables and will be able to calculate for different sizes and wood types depending on what is available localy.