Maximum legal tyre size -Landcruiser 100 series

I've been researching tyres to put on my Cruiser and am more confused now than when I started. Seems to me that the tyre sales people are prepared to let me put anything I ask for on it but what I want to be sure about is that I'm not gonna dud my insurance.

I've got a set of OE GXL rims on it and presently run Dunlop Grand Trek A/Ts [275/70R16]. When I purchased the vehicle it was fitted with the OE Rims and Dunlop Roadgrippers [LT235/85R16]. The tyre placard inside the drivers door lists the tyre spec's as LT235/85R16 108/104N

The owners manual lists both sizes above as being standard for the vehicle. [as I listed in the first paragraph]

I live in WA and think that I am allowed to increase the overall diameter of the tyre by 15mm maximum. [Although I'm not sure that is correct it is my understanding at this time. I stand to be corrected if that is not the case ]

I want to put Cooper AT or ST's on or BFG AT's but what I want to know is what is the maximum size that I can legally put on the thing. I want to go to 285/75-16's just to get a little bit higher off the ground.

Anyone help me with some sort of authoritive answer?

Dept for Planning and Infrastructure [RTA equivalent in WA]not much help as it depends on who answers the phone as to whether or not they even understand what I mean. 3 calls and 3 different answers.

Dave