There's a sting in the tail of lower oil prices, and that's a $100 million hit to government coffers.

Coupled with the fact that government bank deposits are earning less interest than expected, October's promised surplus is looking fragile.

Finance Minister Bill English isn't revising his calculations this far out, but says a low inflation and interest rate environment does put pressure on Government revenue.

"Well, we'll have to wait and see. We'll know in about October this year."

Bill English says the tax take has been running a little higher than expected, but we're talking about pretty small margins.