The Electoral Commission has issued the National Party with a warning over its failure to declare a donation of more than $30,000 on time.
But the commission has stopped short of referring the matter to police, or taking the issue any further.
A spokesman for National said the party has taken steps to make sure "this does not happen again".
"We take our obligations under the Electoral Act seriously and have taken steps to ensure this does not happen again."
"We take our obligations under the Electoral Act seriously."
The commission said it was looking at donations totalling $35,000 from real estate bigwig and regular donor, Garth Barfoot.
He donated $15,000 in May last year, $10,000 in October and another $10,000 in November.
The last donation pushed Barfoot's donation over the $30,000 disclosure threshold and, according to the commission, should have been declared at the time.
Under electoral laws, political parties must disclose donations of more than $30,000 within 10 working days.
That also applies to a series of smaller donations from the same person that add up to $30,000 in a year.
National did not disclose Barfoot's donation until March 31 this year, meaning it was clearly in breach of the rules.
"The commission has written to the National Party reinforcing the importance of adhering to the statutory timeframes and noted that an additional step is being taken to strengthen donation notification processes within the party," an Electoral Commission spokesperson said.
They added that in the past 10 years, parties have filed a late return for donations over $30,000 11 times, including this one.
Three were referred to the police and four parties received a warning. In four cases no further action was taken.
In each case the Electoral Commission looks at all the circumstances including the reason for the failure, the amount involved, when it came to light, and the party's processes for recording donations.