A leaked letter from Treasurer Joe Hockey to state and territory treasurers has confirmed that removing the goods and services tax on tampons and other sanitary items will cost $120 million over four years.

Mr Hockey has confirmed the change remains on the table, though he has cautioned there needs to be unanimous agreement from the states before the change goes ahead.

Protesters against tax on sanitary products on the front lawn of Parliament House. Credit:Alex Ellinghausen

The leak of the letter comes as the Treasurer prepares to address a PricewaterhouseCoopers tax reform forum on Wednesday in which he will set out the six key principles that will guide the government's pursuit of tax reform.

Significantly, Mr Hockey says the government must address bracket creep - the process by which people are pushed into a higher tax bracket because of inflation - as "it is better to leave money in the pocket of the taxpayer".