Property Institute chief executive Ashley Church also sounded the alarm and said "the deck is already stacked" against landlords.
"Auckland is in the grip of a serious housing shortage that affects buyers and renters - and anything that deters investors from providing housing can only succeed in compounding that problem," Church said.
I can attest to that shortage. Having moved last year, only to have our landlord take back possession after six months, I know competition can be intense.
Scores of people turn up for flat viewings, even during working hours.
Applying for a rental can feel like a lottery in some cases and an auction in others, with people offering over the advertised rent to secure the tenancy.
It certainly didn't feel like landlords were the ones on the back foot. And although the concern for tenants is commendable, it is nevertheless misplaced.
That's because Labour's policies won't cause houses to suddenly vanish.
Nor will they remove demand for homes under construction or in the planning stages, given the pace of new builds versus the rate of immigration.
Admittedly, there is a danger that properties get bought by investors who sit on them for capital gain but leave them empty because renting them out is too much of a hassle. But what rational investor would forgo an extra $28,000 a year - the average annual rent for a three-bedroom Auckland home?
The Property Investors' Federation, which King heads, also said an increase in notice periods from 42 to 90 days when a home is being sold would encourage landlords to show tenants the door as soon as they decide to put it on the market.
"First home buyers and other home owners will not want to wait three months before they can move into their new home," it said.
"[This] makes it very difficult to sell a rental property to anyone other than another rental property owner ... properties are more likely to be empty when being sold with no prospect of the sitting tenant being able to stay on should it be sold to another landlord."
A home is most people's most important financial asset, often held for many years. An extra six weeks' wait hardly seems a deal-breaker. For first home buyers, many have already had to wait years to get into a place anyway.
And would setting minimum standards for rentals - requiring them to be "warm, healthy and dry" - scare landlords from the market?
Even if thousands of dollars are needed to get a home up to scratch, any amount is a one-off cost and only adds to a property's value. It is hard to believe an investor would spend (or borrow) hundreds of thousands for a property then refuse to put up a few thousand more, with nearly $30,000 a year at stake.
And rather than being adverse to landlords, many of Labour's proposals are actually in their long-term interests.
If renters are more secure in their position, they are likely to stay in a tenancy for longer, which means a lower rate of turnover and less uncertainty for property owners.
If renters live in healthier homes that are warm and dry this, too, means they are less likely to move on. They, and their families, are less likely to get sick, need to take time off work and run into financial hardship.
The longer people live in houses where they feel secure and healthy, the more likely they are to take care of a property - meaning less maintenance and more peace of mind for owners.
Unless home affordability rates improve, more New Zealanders are going to be tenants for longer.
People on incomes that less than a generation ago would have allowed them to buy their own home could now be renting for life.
New Zealand's rental laws need updating to reflect that shift and - despite what landlords might say - Labour's election promises are a step in the right direction.
In any event, if there's a change of government in a fortnight, tenants won't be the ones to suffer.
