CGT when property later self occupied

My question relates to CGT on a property I bought whilst working overseas, let for 11 years, occupied myself for 18 months when I returned and then sold. The property is in Perth and it appears that over the first 11 years the property probably appreciated at 21,800 per year and yet over the last 2 years at 150,000 per year, roughly 18 months of which we were in residence. A financial advisor advised me to get a sworn valuation as at the occupation date and calculate the CGT on the difference between that valuation and the purchase price, however, my accountant has advised that the tax office applies a straight line calculation method averaging the gain over the complete period then deducting the period in occupation. Could anyone out there please advise me which is the correct method? Thanks