On Thursday they were picketing outside the distribution centre in Wiri.
Meto Fox said they had no other option but to strike.
They had made bargaining claims and given letters from temps describing the toll their work and the lack of employment protection had on their lives.
A union spokesman said there were about 60 people at the picket line, but it was peaceful so far. No police were present.
Foodstuffs North Island chief executive Chris Quin confirmed the industrial action and said negotiations had been taking place for the last six weeks.
"Unfortunately, despite third-party mediation, agreement has not been reached and the Union has decided to strike as of Wednesday 2 August," Quin said.
"As a business we are committed to treating our people fairly and we believe the offer we have put to the Union is fair and reasonable. We have plans in place to minimise disruption to customers."
Quin said the distribution centre employs a labour hire company to provide contractors to support the business at certain times of the year.
- With AAP