DHBs said they have contingency plans at all hospitals to manage the strikes, and that those needing x-rays or scans urgently today should still go to hospital.
The DHBs' spokesman, Nigel Trainor, said DHBs regret the impact on patient services and each DHB is doing what it can to minimise disruption.
He added that any patient whose appointment is affected by the strike will have been notified by their DHB.
DHBs also said that Apex has refused to accept an offer over three years which other groups have accepted.
"DHBs have made or offered settlements comparable with other allied health specialists, nurses, midwives, resident medical officers and many others," Mr Trainor said.
"There is no more money and we will not differentiate pay between sections of the same workforce on the basis of union membership."
Apex national secretary, Deborah Powell, said it's yet to hear from the DHBs about an offer it made last week.
Meanwhile Apex sonographers in Auckland will also strike from Thursday till Saturday.
- RNZ