Which horse is the Prime Minister backing?

Prime Minister Jacinda Ardern has placed her bets for the Melbourne Cup.

She's backing Humidor for her first pick, the only Kiwi horse in the race, and Red Cardinal for her second pick.

Ms Ardern headed to the TAB on Courtney Place in Wellington on Wednesday afternoon, taking a break from the Commission Opening of Parliament.

"I have a patriotic feeling about Humidor," Ms Ardern said.

"I've always been one to back an underdog."

The Prime Minister said the Melbourne cup is "pretty much" the only time she places bet.

Humidor is part-owned by former All Black Mark Carter and trained by Darren Weir, who guided Prince of Penzance to the 2015 Melbourne Cup.

Red Cardinal is an Irish horse trained by Andreas Wohler, who is based in Germany.