Australia's two biggest cities appear to be at the tail end of an investor-led property boom, but rents have not followed.

The Bureau of Statistics inflation figures for the year to March 31 show Sydney and Melbourne rents growing at their slowest pace since June 2006, at just 2.3 and 1.4 per cent respectively.

Brisbane rental growth is the weakest its been in nearly 17 years at 0.7 per cent.

Rents in Perth and Darwin are slumping as the mining boom ends, posting their worst performance on record, with 4.4 and 4.6 per cent slides.

CoreLogic data going back over 20 years reveal an unprecedented fall in national average capital city rents.

If those figures are right, you would think there are a lot of empty properties out there for rent.

However, that is not what the rental vacancy data from Domain, SQM Research or the Real Estate Institute show, except in the really weak markets of Perth and Darwin.

The SQM data for April show a slight fall in Sydney vacancies over the past year to just 1.7 per cent, with Melbourne also lower at 2.1 per cent.

While Perth has blown out from a 3 per cent vacancy rate (generally considered a balanced rental market) to 4.6 per cent, Darwin also actually saw a slight fall to 3.4 per cent.

'Low growth, but high rents'

SQM's managing director Louis Christopher has one theory why vacancy rates and rents appear to have decoupled.

"Relatively low rental growth, but high rents all the same," he explained.

"Very difficult for rents to move up further because they're that high, unless vacancy rates tighten further, or indeed they could well come down if vacancies rise."

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However, western Sydney-based real estate agent Edwin Almeida from Just Think Property has another theory about why rental vacancy rates are so low, especially in Sydney and Melbourne.

"High rise development of anywhere between 150 to 260, 300 apartments in the block, but only 4, 6 or possibly even 10 of those apartments are actually advertised," he observed from his experience on the ground compared to listings on Domain and realestate.com.au.

For the developers and marketers trying to rent out apartments in large, new unit blocks, Mr Almieda said the strategy makes sense.

"It's a prudent exercise by a marketing company or real estate agency not to advertise every single one of the apartments in the block that's available, because what you're going do is you're going to create a downturn in the asking prices," he explained.

New high rise activity may skew vacancies data

Such a practice not only masks the glut of new units for potential renters, but it may also skew rental vacancy data downwards.

That is because most of the data providers rely on the number of properties advertised as the numerator in their vacancy rates.

When approached by the ABC about Mr Almeida's concerns, Louis Christopher decided to check his data for Sydney's Zetland, Green Square region, where high rise activity is very strong.

Given the number of apartments being built in the area, he found the incidence of a few ads representing many properties for rent was very low.

However, he did find a couple of cases where Harry Triguboff's Meriton was advertising just a few apartments out of many available.

"The program that we generate [to scan ads], if Harry, in those two developments, has a number of properties there which are available for rent and are unoccupied, we would be missing those," Mr Christopher acknowledged.

He said the same would apply to other large developers who can afford to build apartment blocks and drip-feed their units for sale, renting them out in the meantime.

Data error 'maybe a tenth of a per cent'

However, Mr Christopher said very few developers can afford to do that, so the numbers are likely to be small.

"New properties coming into the market, and then out of those new properties where it's just developers who have the capital to be able to advertise for rent themselves, really represent a very small section of the market, I would argue that would be less than 1 per cent of the rental market," he added.

Domain's chief economist Andrew Wilson agreed that the impact on vacancy rate figures, such as those provided by his firm, is likely to be small.

"They would only have an impact of perhaps a hundredth of a per cent on the actual vacancy rate, maybe a tenth of a per cent," he said.

For lease and for sale signs proliferate outside an apartment block in the western Sydney suburb of Westmead, January 2016 ( Audience submitted: Edwin Almeida )

Both analysts also added that, because the way their figures were calculated had not changed over a long period of time, even if the vacancy rates were understated that miscalculation would be consistent over time.

"These things aren't absolute numbers, it's about the trend and it's about the robust nature of the comparisons between capital cities," added Dr Wilson.

The big danger for the reliability of the rental vacancy data is that the recent trend has been strongly towards these large apartment developments where this problem potentially arises.

That could make the data inconsistent over time, and across cities which have different levels of high rise activity.

Louis Christopher said it was something he was now watching.

"This is something that we'll be keeping a closer eye on," he said.

"At this stage though it's not something that's really causing us a lot of alarm, if it does become an issue we'll certainly be making adjustments if we need to do so."

Rival data to hit the market in July

Rival property analysis firm CoreLogic RP Data is planning to launch its own rental vacancy data in July.

While it is also based on ads, the company's research director Tim Lawless said the figure will measure how long advertised properties are remaining vacant, rather than estimating what percentage of properties are empty.

"If we're quoting a 4 per cent or a 5 per cent vacancy rate in a particular region that means that property is likely to remain untenanted for 4 or 5 per cent of a year," he said.

Mr Lawless said that should largely address the problem of unadvertised rental properties, because it is comparing each advertised property to itself.

"It kind of removes some of that bias you might find from un-reporting or properties that aren't listed for rent in the marketplace," he said.

So far, Tim Lawless said CoreLogic's data shows vacancy rates remain relatively low in most cities and are not spiking higher.

However, both he and Andrew Wilson speculated that this may be because foreign buyers are soaking up a lot of the wave of newly built apartments in Sydney and Melbourne, with many left empty.

Alternatively, weak rental growth may just be a sign, as Mr Christopher argued, that renters simply cannot afford to pay more.