It takes a short drive around Sydney to recognise that building approvals have soared. It is a secret that cannot be hidden.

NSW experienced two consecutive fiscal years of record high unit approvals (referring to all dwellings except for houses), the peak being during the 2015-16 financial year, according to ABS trend estimates.

Many think it is an apartment tsunami, but should we really be negative towards urban densification? The level of construction is not surprising given the strength of population growth and, of course, the issue of housing affordability must be mentioned.

September 2002 was a construction turning point. This was the first time that unit approval rates in NSW surpassed the number of house approvals. For a short time afterwards, housing approval rates did take the lead, but consistently since February 2012, more units have been approved in NSW than houses on a month-by-month basis.

Recently, the surge in construction approvals has softened. During the first eight months of 2017 there were 26,778 unit starts approved in NSW, plummeting 16.9 per cent compared with the same period in 2016, and down 6.3 per cent compared with 2015.

Unit approvals did make a modest rebound by 1.4 per cent from July to hit 3476 in August, the highest monthly result since October 2016.

Approval data provides insight into the potential pipeline of stock, however not all approvals will translate into completed dwellings. There can be delays between the approval and build-start, a completion lag, and changes to the conditions in the specific sub-market (or to the overall dynamics of the market) can hinder or even stop a development from proceeding.

ABS statistics reveal that there has been a decline in the number of houses approved in NSW where construction has not commenced.

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The scenario is the reverse for units. Comparatively, NSW has recorded a high number of unit approvals where construction has not commenced, signifying that a large number of developments are on hold.

Data on the value of residential building work completed in NSW outlines actual additions to property stock. The 2016-17 financial year saw the biggest economic boost at $12.6 billion worth of unit construction, with the June quarter providing the largest quarterly dollar injection on record. This was a hefty jump of 18.5 per cent from the prior fiscal year.

Dr Nicola Powell is a data scientist at Domain Group. Tweet your questions to @DocNicolaPowell.