Unions are warning Wellingtonians to "get used to walking" as they prepare to take action that will bring buses to a standstill.

Drivers and supporters will set up picket lines in Wellington and the Hutt in June and July, to prevent any buses operated by Tranzit from leaving depots in protest over contracts.

It's set to disrupt tens of thousands of commuters, with around 100,000 bus trips taken each week in Wellington.

"We intend to picket them so that nothing operates in this city without there being justice for people who provide public transport in Wellington," Union advocate Graham Clark said.

"Citizens of Wellington, get used to walking."

Mr Clark issued the threat in front of Prime Minister Jacinda Ardern and Wellington mayor Justin Lester at an event at Trades Hall in Wellington last week.