Westpac New Zealand said it had an offer of September 2018 bonds aimed at raising up to $800 million.

The offer, which closes on Friday, follows last year's successful five-year bond issue, which had to be expanded to meet investor demand.

Westpac treasurer Jim Reardon said the issue would "re-tap" last year's issue and would be mutually interchangeable with it.

The latest addition to the bank's bond offering would give investors a deeper liquidity pool to trade bonds.

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Reardon said that despite uncertainty in world markets, the current environment offered a "relatively clean" window of opportunity for the bank to raise funds.

He said the introduction last year by the Reserve Bank of loan-to-value ratio (LVR) restrictions - aimed at curtailing high-risk home lending - had created some uncertainty.

"The big unknown for all the banks at the moment is just how much loan growth we are going to see through the next 12 months," Reardon said.

There was also uncertainty about how much loan growth a "fairly buoyant" local economy would create.

"But from our point of view, we would rather be well-funded going into that," he said.

Westpac is rated AA minus by Standard & Poor's, Aa3 by Moody's Investors Service and AA minus by Fitch Ratings.