Advise on bearers and Joists

so what would the maximum span be for the posts be using two 90 x 45 for bearers?

My span tables don't show 90x90 but a 90x70 in F5 will span up to 1200 or 1500 in F8.

1500mm max between posts using two 90 x 45's (or 90 x 90 ) and 1500mm for the joists ( 90 x 45). This is max though and although will do the job and is legal it will have a little spring in it . Better th bring them all in a little if you can.

I had a look at the tables I have and don't show 90x90 in F5.

Bearer span also takes into account the joist span. Longer joist span = less bearer span, as each bearer is now supporting more floor.

Try this for a program for workiong out spans

https://www.woodworkforums.com/showthread.php?t=55014

Yes, the spans I gave were for a bearer spacing of 1800, but given the tight space between the pool and the retaining wall, he's not likely to exceed that. Possibly on the downhill side though - however I'd suggest a much deeper bearer for that portion of the deck.