The top PIR rate of 28% has a tax arbitrage wedge against the top marginal income tax rate of 33% of 5% as PIE income taxed at the PIR is generally a final tax and therefore so long as you use the correct PIR you don't have to include the PIE taxable income in your tax return. It's generally accepted that wedge is partly there to encourage savings in the funds sector potentially for retirement savings.
The prescribed investor rate for a person who is an investor in a multi-rate PIE is set out in Schedule 6 of the Income Tax Act 2007 and the 28% is the default rate. The other rates 10.5% and 17.5% apply if in either of the past 2 income years a taxpayer had earned taxable income below a threshold or their taxable income and PIE income was below a threshold.
http://legislation.govt.nz/act/publi...tml#DLM2918738
In the event that the top marginal tax rate would increase to 39% then the government could do nothing to Schedule 6 AND create a larger wedge between the 28% Top PIR and the 39% marginal tax rate but that could reduce the income tax that they hope to collect from the rate hike - but in IMHO an 11% arbitrage wedge is sufficiently large that a new PIR could be specified say at 34% (keeping the 5% wedge) where taxable income and attributed PIE income was over the $180,000 threshold in the past two years. It would also be a windfall if that 34% is the default unless an investor selects their correct and lower PIR.
In terms of paying the marginal rate - given the data I've seen on the distribution of taxable income and the somewhat uneven concentration of the number of taxpayers who have taxable income just below the $70,000 top rate bracket tier then I think it's human nature that we will see taxpayers legally arranging their affairs to reduce their taxable income below that proposed $180,000 tier.