G'day rossstewart, welcome to GPS Power.
You must mean you downloaded all the rar parts 11 times i guess which would be quite well over 50GB. Actually, the 'in 11 rar parts' was a typo, it's 12 parts all up, 11 of 400MB and the 12th of 157.17MB making ~4558MB all up.
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When extracted the ISO file is 4,779,573,248 bytes [~4.78GB or ~4.45GiB] which is of course why it won't fit 'as is' on a 4.7GB [~4.38GiB] SL DVD. Tip for removing the POIs to allow it to fit is also quoted in the initial post [trannhan81] but it is risky to use the DVD as also described without LOADING.KWI because of the risk of a power-down wiping the volatile memory and the unit's firmware with it. Full LOADING.KWI is needed on the disc to re-write it. The method you describe is better as there is never a need to have the AU POIs in a Kiwi car or the NZ ones in an Oz car.
Thanks for your post.