Is it time to put the brakes on property investors, again?

They settled on placing a 10 per cent cap on growth in loans to property investors, forcing banks to tighten lending standards, and a crackdown on interest-only loans.

It was never officially aimed at slowing house price growth – but the boom in Australia's two biggest cities was an essential part of the picture. At the time, Sydney's market had leapt by 31 per cent in the previous two and half years, Melbourne's by 17 per cent.

Two years on, and growing number of experts are asking out loud: is it time for our financial watchdogs to consider turning up the dial once more?

Since those December 2014 policies were unveiled, Sydney house prices are up another 29.9 per cent, and Melbourne's are up 27.4 per cent, CoreLogic data show.

Bank lending to property investors is also re-accelerating, after falling sharply in 2015-16. Loans to investors now make up 49.6 per cent of new mortgage lending, not far off historic highs.

How the regulators respond to the latest investor loan boom is shaping up as the more interesting questions in domestic banking this year – which could have a big impact on the broader real estate market.

Reserve Bank of Australia governor Philip Lowe acknowledged the rise in his statement after Tuesday's board meeting, pointing to "stronger demand" from investors.

Lowe also noted that supervision of the banks had resulted in stronger lending standards, saying some banks were taking a "more cautious" approach to lending in parts of the market. Economists said this could address some of the fears about the property market's re-heating that the RBA may have held.

To be sure, borrowing by landlord investors is still well below the 10 per cent growth cap, with an annual rate of 6.2 per cent. That's a far cry from the peaks of 10.8 per cent in June 2015.

But over the last quarter, the annualised pace of investor credit growth  surged to 9.1 per cent, as prices also took off. House prices have jumped 16 per cent in Sydney in the last year, and 11.8 per cent in Melbourne.

Why might all this be a concern?

Well, at face value it looks suspiciously like last year's interest rate cuts have reignited the two hottest markets in the country, despite the previous crackdown on housing investors.

The RBA has previously said it sees geared property investment as more "speculative," meaning some investors are simply placing leveraged bets on prices going up.

This suspicion is reinforced by other figures showing rental returns from property investment are not keeping up with the price gains. CoreLogic last week said rental yields - the annual return from rent as a percentage of property prices - had fallen to fresh record lows.

The RBA and the Australian Prudential Regulation Authority have always said their goal is not to manage house prices, and that's fair enough.

But as credit ratings agencies have repeatedly highlighted, fast-rising prices are also a source of risk to the banks, as about 60 per cent of all bank lending is for residential property.

So, what might be done?

Independent economist Saul Eslake says that if you see the rebound in investor lending as a concern, there are three options on the table.

One would be to raise official interest rates - but don't count on that. When inflation is still well below the RBA's target, the market is firmly in agreement that a rate rise is a long way off.

A second option would be remove some of the tax breaks that help make property investment such hit with Australians in the first place, such as scaling back negative gearing or the concessions on capital gains tax.

But once again, don't hold your breath for that to occur under this government.

So that leaves a third possible change: forcing banks to further restrict their lending to property investors, as occurred in 2015, resulting in bank slashing how much they were prepared to lend.

Eslake isn't sure that regulating our banks should be a "de-facto" instrument of monetary policy, but even so, he is by no means the only market raising this as one option for regulators to consider.

AMP Capital's chief economist Shane Oliver has also pushed for lowering the cap to 5 to 7 per cent, which is still faster than household incomes.

UBS banking analyst Jonathan Mott said in a note last week the resurgence in lending to property investors was a "concerning development," even though it's good for bank profits. If lending to landlords keeps accelerating, he says calls for tougher rules on bank lending are "inevitable."

Such a toughening might mean forcing banks to assess borrowers more stringently. Or it might result in banks raising interest rates for property investors but sparing owner-occupiers, as the big four banks did in late 2016.

The experience of the last few years shows these sorts of changes can at least temporarily take some heat out of the housing market, without inflicting broader economic fallout.

A final option, of course, is for regulators to keep a vigilant eye on the market, without taking action.

It is possible that credit growth will slow down on its own accord, as the high cost of housing limits how much further prices can rise.

But after all their talk about managing risks in the housing market in recent years, you'd have to wonder how willing regulators will be to take that chance.