If he puts Labour in power he will want to be confident that the fiscal plan it published before the election can accommodate state-sector pay rises of this magnitude.
Labour is close enough to the teachers' unions to have been aware a claim of this scale was being prepared and make provision for it.
Both sides of politics agree teachers' pay in New Zealand is too low. They disagree on whether the solution is to raise rates across the board, as the unions demand, or to pay more for performance and for subjects short of teachers.
It is an important issue, which can too easily expose teachers generally to damaging attacks.
Overall they do a fine job. New Zealanders can send their children into any school in the country and be confident the staff will be caring and dedicated and the school will be a stimulating, inclusive environment.
If some of the staff contribute less than others to pupils' progress, only principals and colleagues generally know who they are.
And most principles and colleagues appear to share the unions' philosophy that performance pay would be damaging to collegial co-operation.
Other professions appear to co-operate well enough with variable incomes but if teachers want standard rates, nationally negotiated, they cannot also expect to be paid at the rates good teachers deserve.
They are aiming to increase the top of their basic pay scale from just under $76,000 a year to $87,000, which does not sound excessive.
The best deserve considerably more, and schools probably need to offer more to attract teachers of science, maths, Maori language and other subjects with a shortage.
Public funds for education are not unlimited. Every additional dollar paid to an underperforming individual is one less that can help recruit and retain those who provide more value.
It is an old problem and there is a real prospect of industrial action in the new year if the unions do not get their way.
It is a prospect that will haunt Peters and the parties vying for his favour. It is a reality check for them all.