Unclaimed Div 43 is not added back, except for the portion of Div 43 that you omitted to claim for the current year and any recent years for which you're entitled to re-open your assessment. (Broadly either 2 or 4 years, depending on your specific circumstances, and of course you can then claim it back for those periods anyway)
If you never claimed Div 43 because you never received the appropriate information on original construction cost, then no add back is necessary.
Here's the link to the ATO website covering this:
http://www.ato.gov.au/individuals/c...002/026/017/004&mnu=1051&mfp=001/002&st=&cy=1
Of course, as with all tax matters, nothing is ever straightforward, (particularly with CGT!) so please consult with your accountant before making any decisions.
Rob G is correct to say that you're generally better to claim the capital works deduction (Div 43), assuming that you're paying tax at 30% or higher.
If you're paying tax at 15% and the capital gain pushes you into the 40% or 45% bands then this might not hold true.
For example, if you've claimed $10k of Div 43 deductions at 15%, you've received $1,650 cash back (incl medicare). If the capital gain is then taxed at 40%, that $10k deduction will increase your CGT by (10k * 41.5% * 1/2) = $2,075. (incl medicare)
This is not as extreme a situation as it might appear. The 30% band is currently $46k, so a capital gain of $100k, assuming held for more than 12 months, would lead to a taxable gain of $50k, taking someone from the top of the 15% band to the bottom of the 40% band.
Cheers
Jonathon