I think it depends on what you use the additional borrowings for. If they are spend on buying/renovating the investment property then I think that interest on the top-up is allowable against rental income. If the money is used for some other purpose then I don't think that it is.
subject to the normal limits
What limits? I thought that 100% of interest on loans used to buy/renovate an investment property could be offset against rental income (and not other income as far as I know) without limits?