A mortgage broker says its hard to know if rival major banks will follow in the foot-steps of Kiwibank and offer a sub-4 per cent rate home loan rate.

The government-owned bank is from today offering a rate of 3.99 per cent for a two year mortgage - the lowest rate the bank has offered since it began in 2002.

The special is only available to borrowers with a 20 per cent deposit and has a short timeframe of just two weeks.

Kiwibank is the first major bank to offer a rate under 4 per cent.

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HSBC is currently running a special of 3.95 per cent but borrowers must take out a mortgage of at least $500,000 or have $100,000 in the bank.

Bruce Patten, a mortgage broker with Loan Market said none of the major four Australian-owned banks had followed HSBC's offer and it was hard to predict if they would also match Kiwibank.

"Even though Kiwibank is a significant player they are still a small part of the market.

"It's too early to tell if anyone else will follow suit."

Patten said the low rate offer by Kiwibank was a surprise given it and other banks did not cut their rates by much after the official cash rate was cut unexpectedly on March 10.

But it could pre-empt a rate cut at the Reserve Bank's next announcement on June 9.

The official cash rate is currently at a record low of 2.25 per cent.

Patten said he expected there to be a further rate cut at that announcement on the back of concerns expressed by the Reserve Governor this week about a deterioration in economic conditions around the globe.

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But Kiwibank's move also comes in a week where the Reserve Bank released its financial stability report expressing concerns about the level of housing debt New Zealand has.

"The Bank remains concerned that a future sharp slowdown could challenge financial stability given the large exposure of the banking system to the Auckland housing market," Reserve Bank governor Graeme Wheeler said.

Both the Reserve Bank and the Government have talked about further measures to control household debt including the possible introduction of debt to income caps for home loans.

Patten said if debt to income caps were introduced low earners and first home buyers would be those most affected by the change.