Prosecutors said Danish investigators who looked into the death had no idea of the potential windfall that McPherson was set to receive.
The prosecution claims McPherson "resented" Leeson's family for "sitting on wealth" and had taken out various insurance policies on her life, collectively holding a value of more than $6.7 million.
The court also heard that McPherson, originally from Auckland, was keeping secrets from Leeson, whom he married in 2014 in a lavish ceremony at a historic British castle.
David McLachlan QC told the jury that the "big secret" was the insurance policies, but also told them that forensic handwriting experts would prove that McPherson forged her will to make himself the beneficiary.
By 2016, McPherson was paying nearly $900 a month in premiums for the life insurance policies. Prosecutors said he never missed a payment despite heavy debts.
McLachlan also told the jury that McPherson, who grew up in Auckland with his parents and two sisters, told his wife that he was an orphan to avoid discussing his past.
He had also trained to be a pilot, keeping his training secret from Leeson by claiming to be working if she called during his flying lessons.
In June 2017, McPherson and Leeson headed to Norre Nebel in coastal Denmark for a mini-break, despite Leeson hating the seaside.
Prosecutors also claimed Leeson hated the pool - but it was there she would die.
After three days on holiday, Leeson drowned in the pool of their rented cottage.
McLachlan told the jury: "The prosecution case is, that while at first glance it appeared that her untimely death was an accident, the evidence will show that it was not.
"It was a sinister planned killing and the person responsible for her drowning was none other than her husband Donald McPherson.
"The motive for the drowning was the oldest and simplest one in the book. It was financial.
"He stood to gain a vast fortune by her death. This was something which was not known by the Danish authorities in the immediate aftermath of Paula Leeson's death."
The trial continues.
