A second major bank is set to join the sub-4 per cent mortgage war which was reignited last week after a lull over Christmas.

From Monday Kiwibank will offer a two year fixed term rate of 3.99 per cent. It comes on the back of Westpac's 1 year fixed term special of 3.99 per cent which is due to end on Friday (February 1).

The Kiwibank rate, which is down from a 4.19 per cent special rate, is only available to those with equity of at least 20 per cent. It is available for two weeks.

Those with less than 20 per cent equity face a much higher rate of 4.94 per cent.

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Last week mortgage brokers said they would not be surprised to see other banks follow Westpac's lead in offering a sub-4 per cent rate.

John Bolton, managing director of Squirrel Mortgages, said wholesale rates had fallen recently which had taken the pressure off bank margins after rises last year.

"Wholesale rates have come back a bit."

In November last year, ANZ - the country's largest bank - offered a one-year fixed term rate of 3.95 per cent - the lowest offered by a major bank since just after World War II.

Other banks followed with ASB and Westpac matching it and BNZ offering 3.99 per cent over two years fixed.

But by December 4 three of the four majors had put advertised rates back up over 4 per cent.

Still, the short-term offer appears to have been worth it.

Figures from ANZ show around 15,000 customers took advantage of the special.

"This helped grow our market share in November, contributing to more than $500 million in new home lending," said a spokeswoman for the bank.

She said the offer had made November its biggest month for home lending since August 2016 with total home loan growth 4.5 times higher than around the same period in 2017.

"Calls to our contact centre also spiked 80 per cent in the following days too."

Karen Tatterson, a mortgage broker with Loan Market, said anyone due to re-fix their mortgage in the next 60 days should try and lock in an offer and those who were due to end their fix term in the next six to nine months should also weigh up the cost of breaking their mortgage to re-fix it at a lower rate.

Bolton said it was always worth negotiating to see if other banks they would match the rates.

"You will find that by and large they match each other."