In Auckland sales fell by 733 properties (30.6 per cent), from 2,399 sales in July 2016 to 1,666 in the same month this year.
Sales by auction had also experienced a decline, with 14 per cent of the 767 sales, sold at auction, within the same period, compared to 23 per cent of all sales in July last year.
REINZ Bindi Norwell said there was a perfect storm of factors contributing to the property market's recent slowdown.
"When you throw in an election, winter, school holidays and one of the wettest Julys on record, it's little wonder the number of properties sold last month fell so significantly."
Trade Me Property's data showed a similar picture of a market where it was taking longer to sell - particularly in Auckland and Christchurch.
In the country's largest city, houses were listed on the site for an average of 41 days throughout autumn, compared to 28 days last autumn and 27 in 2015.
In Christchurch houses were on the market for an average of 45 days, compared to 39 during the same season last year.
Wellington was the only one to buck the trend, with sales times almost halving across the last four years, from 50 days in the autumn of 2014 to 28 this autumn.
However these figures risk understating the actual time properties took to sell, as they did not take into account time spent on the market through previous listings - with some listed on the property site for years.
Details of those that have taken years to sell could not be divulged in the Herald due to privacy concerns.
Trade Me head of property Nigel Jeffries said as the weather began to warm, the market would not necessarily follow suit.
He said spring was one of the most competitive times of the year to sell, so properties would get less views and stay on the market longer than in other seasons.
"Spring is a great time because everything is starting to look greener and more inviting, but from what we've seen you shouldn't hang your hat on the 'only sell in spring' mantra.
"As the weather heats up in summer, and both buyers and sellers move into holiday mode, we see competition cool in the property market."
Jeffries said winter was the best time to sell if you wanted to do it quickly.
However, for those unable to wait for a particular season to sell he had a number of tips.
These included knowing the market, setting the right price, sprucing up the property so it presents well and targeted advertising.
"The challenge is to get someone else to fall in love with your property and achieve this over a two to three week period.
"This is where well-honed sales skills come to the fore. A great agent lives and breathes this stuff so take their guidance."
Tips to get your house sold quickly, from Trade Me Property Head Nigel Jeffries
Price it right:
There are a number of online tools where sellers can evaluate what their property is worth and what it might be most likely to sell for. A good agent should also be able to advise on the likely sale value and best asking price.
Presentation:
Declutter, arrange the furniture well, tidy gardens and even the delicious smell of baking during an open home - it's the little touches that count
Targeted marketing:
Know who your potential buyers are and design a plan with them in mind. Sellers will also need to know what the competition is and don't be afraid of spending money to make money. The rule for the ideal marketing spend is 1 per cent of the value of the home.
Paperwork
Have the right information ready for potential buyers, including the LIM, Title, Valuation and Building condition report and a description of what the suburb offers.
Days on market Autumn 2016 to Autumn 2017
Data sourced from Trade Me Property
Auckland
2014: 39
2015: 27
2016: 28
2017: 41
Christchurch
2014: 40
2015 41
2016: 39
2017: 45
Wellington
2014: 50
2015: 47
2016: 25
2017: 28