The 6 per cent margin is $43,200 on a median priced Auckland home.
Auckland City had the biggest differential, with auctions delivering prices 20 per cent above CV compared with private treaty (11 per cent).
The North Shore had the smallest margin - 15 per cent over CV for auctions compared with 12 per cent for private treaty.
Loan Market mortgage adviser Bruce Patten said auctions were delivering massive sale prices for vendors amid a shortage of properties and surging demand from cashed-up buyers.
A Beachlands home that went to auction on Monday with a $1 million reserve sold for $1.22 million as bidders slugged it out.
"You'd be pretty happy if you were effectively prepared to accept a million and got an extra $220,000."
But Mr Patten knew clients who had spent up to $5000 on builders' reports, LIM checks and title searches before securing a property.
The due diligence costs affected buyers "across the board", though first home buyers were worse off as many also required registered valuations to obtain finance.
As the costs mounted, some buyers were skipping pre-purchase checks to save money, putting themselves at huge financial risk.
Nationally a record 2267 properties were sold by auction last month, more than a quarter of all sales and an increase of 604 on March 2014.
The numbers were dominated by the Auckland market, where nearly half of all sales were by auction.
Auckland's biggest realty firm Barfoot & Thompson is driving the trend.
Managing director Peter Thompson said 55-60 per cent of the firm's listings were auctions, and 80-85 per cent were sold "under the hammer or within two days thereafter".
With prices accelerating so steeply, properties were selling for well above CV.
Auctions were the best way to achieve "maximum dollar" and more reliable than trying to predict a property's value, he said.
"If you can get one or two people bidding against each other, it will generally push the price above what they could get if they were just doing it by fixed price."
However he admitted up-front due diligence costs were tough on buyers and warned against skipping crucial pre-purchase checks, particularly around a property's physical state.
Real Estate Institute chief executive Colleen Milne said the current Auckland market was conducive to the auction process.
"With the lack of listings, some people are prepared to pay over and above perhaps what the value of the property is just to obtain a residence."
But not all agencies favoured auctions and lower value properties in parts of South and West Auckland targeted by first home buyers often had advertised prices, she said.
Buyers told reserve price
Vendors usually keep their reserve auction price under wraps.
But a Pt Chevalier property owner has turned the process on its head - disclosing the $940,000 reserve to potential buyers before it went under the hammer.
Professionals agent Lawrence von Sturmer sold the renovated three-bedroom bungalow last week.
The property, at 42 Walmer Rd, has an $840,000 CV but was snapped up for $1.2 million. It means the owners made $260,000 more than they were prepared to accept.
Mr Sturmer said the novel selling technique meant bidders knew they could afford the reserve price before paying for building reports or registered valuations.
"If you've got an owner who knows what they're willing to accept and an agent who agrees that the value is in the correct price range, why not tell everyone, 'This is what you need to buy the home'.
"You can spend thousands of dollars on due diligence, go along to the auction to bid, then find the reserve was over your financial cap."
He said the selling approach brought more people out of the woodwork on auction day and ultimately helped lift the sale price.
The cross-lease home sits on a 695sq m section. It has one bathroom and no parking. The house has character features, open plan living area and a built in gas fire and Smart TV. It also comes with new carpet, polished wooden floors and alarm system, and is handy to the shops, transport, Motat and Auckland Zoo.
