While it is not expected that you will be an expert on tax when it comes to property investing, you should have a broad knowledge of the tax implications of any steps your client is likely to undertake and any strategies they use.
Interest on a loan for investing purposes is only tax deductible where it is taken out for an income producing asset. This fact also applies to any costs incurred when buying such an asset, which means that any costs incurred prior to the ownership of the asset, and prior to any income being generated, do not provide an immediate tax deduction. Such interest and/or costs become a part of the cost base and may, at the time of eventual sale, reduce the capital gain made and so the capital gains tax.
In addition, the purpose of loan funds is what determines tax deductibility of all, not the security of the loan. This has serious implications for those who may use their own cash for a deposit, but later recoup that cash from a 100% loan, which is advanced upon settlement. The interest on the portion which is being refunded to the purchaser to repay this initial deposit may be declared non-deductible, since in reality that portion of the funds was not used to buy the asset.
As a wise property advisor, you need to ensure that the advice of a skilled accountant is sought for all questions about tax deductibility.