The Tax Working Group's recommending a broad tax that goes beyond residential rentals.
The Finance Minister says modelling in the group's report suggests there may be some impact on rents.
But Robertson says there's also the lived experience of other jurisdictions, where it did not have that effect.
He says that's an issue the Government will assess.
Group chair Sir Michael Cullen says the fear a capital gains tax would discourage investment, is not backed up by evidence.
He says it hasn't happened in other jurisdictions.