Five fire crews battled a "well-involved" fire in the clubrooms of an Auckland sports facility this afternoon.

Northern Fire Service shift manager Scott Osmond said the fire at the Windmill Park netball and tennis courts in Epsom, is being treated as suspicious.

Fire crews are still at the scene with a fire investigator, looking into what caused the blaze.

Osmond said he was not aware of any people being inside the clubrooms when the fire broke out.

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The Windmill Rd netball courts was one of two venues for Auckland Netball's summer league which ended on November 23.