"It will be interesting to see what the results look like", Mr Key said this afternoon.
"We'll be putting them next to the million people that voted for us and for this policy in the last election."
Justice Minister Judith Collins said the Government was required to hold the referendum by September 2014 but had had decided to conduct as soon as practicably possible.
"Completing it before the traditional holiday period begins will help us get maximum voter participation."
The Government was required to hold the referendum after the Labour and Greens led Keep Our Assets coalition secured 327,224 signatures in a petition.
Greens Co-leader Metiria Turei said the referendum was "a victory for democracy" and called on the Government to halt assets sales until it was completed.
She said Mr Key had not only refused but had "tried to sell as much as he can before the public has its say".
"The Prime Minister believes he has a mandate for these sales; that mandate will now be tested," said Mrs Turei.
Labour Leader David Cunliffe said his party was pleased Mr Key had announced a date for the referendum, "but the timing is cynical".
"He wants the referendum buried before summer beer and barbeques washes it away.
"We expect voters to send the Government a powerful message about its partial privatisation programme. The result will kick the last leg out of John Key's moral authority to sell public assets."