GWENT councils spent more than £11.5 million on supply teachers over the past three years.
Newport council spent the most totalling £4,709, 292.95 since 2009.
Figures obtained under the Freedom of Information Act show the figure has risen from £1,156,746.03 in 2009/10 to £1,666,532.44 in 2010/11 and £1,886,014.48 this year.
The amount paid out by Blaenau Gwent has risen from £878,417 in 2009/10 to £955,913 the following year to £1,001,878 last year.
Monmouthshire council spent £573,107 three years ago, £636,969 last year and £819,536 in 2011/12.
Torfaen council said it does not hold the information as agency staff are directly employed by schools.
Despite being the biggest of Gwent's local authorities with 90 schools, Caerphilly spent the least during the same period and figures have more than halved in that time.
This is down from £1,007,207 in 2009/10 when it employed 332 cover staff, to £584,894 for 268 supply teachers in 2010/11 and £420,125 this year when it employed 184 agency staff. All together the amount spend in Gwent was £11,587,338.95.
Individual schools hold records for why specific cover is needed but most councilÕs including Newport said it was generally down to staff sickness and time spent away on training courses.
A spokeswoman for Blaenau Gwent council said absence levels had reduced significantly since the adoption of the current sickness absence policy in schools.
Caerphilly council failed to respond to a request for a comment.