Errr they can tell you market value in case you're mods go over 25-40% of that value where they'd reconsider insuring you.
You are confusing market value for an assessed pre-accident value which has several inputs, the market being one of them. Redbook is good for getting an idea if you are concerned but every insurer is different.
I know RACQ uses valuers and have a tendancy to overvalue.