After being alerted to the transactions by the Herald, the Real Estate Agents Authority has launched its own probe.
Chief executive Kevin Lampen-Smith said the watchdog was looking into the sales.
"In general terms, it is legal to on-sell a property that you have signed a sale and purchase agreement for before settlement is complete. However, a subsequent sale and purchase agreement would have to be conditional on the settlement or fulfilment of the original agreement.
"Real estate agents are required to act in their client's best interests. If a licensed real estate agent is acting for the first vendor, they need to ensure that they are seeking and obtaining the best price possible for their client."
Lampen-Smith said buyers should always check that the name of the owners on the title deed match the names on the sale and purchase agreement.
Property records analysed by CoreLogic show one of the properties was sold three times in four days with its price jumping by $150,000.
The modest bungalow was originally owned by Ben and Jessie Church, who sold it through Mangere Bridge Realty on August 4 for $1.9 million to Jia Liu.
Jia Liu flipped the property the same day to Sha Liu, who on-sold it four days later to Treasure Plus Ltd for $2.05 million.
It is not know whether the two on-sales were handled privately or if they involved licensed real estate firms.
The other property is a historic homestead once owned by Auckland department store owner George Courts.
Its price nearly doubled from $1.45 million to $2.8 million between February and August last year, with investor Hua Wu making nearly $1 million in capital gain.
Wu bought the house on February 14 from previous owner Jennifer Woodman in a sale handled by Ray White's Mangere Bridge branch before selling it six months later to Sha Liu for $2.4 million.
Sha Liu flipped it four days later for $2.8 million - a profit of $400,000.
It is not known whether the two on-sales were handled privately or done through licensed realty firms.
Ray White's head of New Zealand agency operations, Graeme Fraser, said in a statement the company had no information regarding the Coronation St properties beyond the only sale it was involved in on behalf of its vendor, Jennifer Woodman, on February 14 last year.
Woodman was shocked to learn her old property sold for nearly twice what she received for it just six months later and said she had hoped it would not fall into developers' hands.
The old homestead is not listed for heritage protection, but Auckland Council is now carrying out a heritage assessment on the property.
