Primary teachers and principals will hold a secret on line vote this week on whether to extend a half day strike next month to a full day

Their union, the NZEI, says they want to send strongest possible message to the Government.

Teachers have already decided to stop work from 1.30 to 4.30 on the afternoon of August 15 to support their claim for more money.

The Ministry of Education has offered increases of between 2.2 and 2.6 per cent a year for three years.

That would take the beginner rate to just over 50 thousand dollars a year, rising to 55-thousand in 2020..

But that's a far cry from the 16 per cent teachers say is needed to retain and recruit staff.