Mr Spencer said that the Reserve Bank was now considering tightening restrictions on loan-to-value ratios (LVRs) for investors further.
Under current LVR limits, banks must demand 30 per cent deposits for a mortgage secured against an investment property in Auckland.
LVRs of 20 per cent also apply to non-investors and the rest of the country.
Mr Spencer did not say how those limits could be changed.
He also said that debt-to-income ratios could also have a role to play.
This measure, if introduced, would stop people from borrowing too much relative to their income.
Mr Spencer said it was a new instrument and would need approval from the Finance Minister Bill English.
"Further investigation of this option will be undertaken," he said.
The introduction of debt-to-income ratios in New Zealand would be a controversial move - opponents say that such a policy would hit first home-buyers the hardest.
However, the measure could also be used to target investors.
It is already applied in the United Kingdom, where most buyers cannot get a mortgage higher than 4.5 times their annual earnings.
Mr Spencer said that while demand for housing had been strong, supply had been constrained by rigid planning rules, reluctance by communities to move to greater-density housing, inefficiency in the building industry and constraints around resource consents.
A broad range of initiatives was necessary to increase supply, he said, especially in Auckland.
"The Reserve Bank has no direct influence over supply, but can influence housing demand through the credit channel. In this regard, we see the Reserve Bank as part of a team effort.
"A dominant feature of the housing resurgence has been an increase in investor activity, which increases the risk inherent in the current housing cycle."
Quotable Value (QV) figures released this week showed that national house values had risen by 5.6 per cent in the last three months - the fastest rate in 12 years.
Auckland's average house price is now $975,087 - up 16 per cent on last June.
Following the release of the QV stats, Prime Minister John Key urged the central bank to target investors, telling them to "get a move on".
The prospect of stricter lending rules was cited as a reason for rising investor activity, as buyers rushed to snap up homes before any lending changes.
The Reserve Bank also made strong comments on migration, saying that Government should consider reviewing its immigration policy.
Mr Spencer said record net migration was driving increased housing demand, and migration flows were now becoming more dispersed, affecting areas outside Auckland.
"Like taxation of investor-owned housing, migration policy is a complex and controversial issue," he said.
"However, we cannot ignore that the 160,000 net inflow of permanent and long-term migrants over the last three years has generated an unprecedented increase in the population and a significant boost to housing demand.
"Given the strong influence of departing and returning New Zealanders in the total numbers, it will never be possible to fine-tune the overall level of migration or smooth out the migration cycle.
"However, there may be merit in reviewing whether migration policy is securing the number and composition of skills intended.
"While any adjustments would operate at the margin, they could over time help to moderate the housing market imbalance."
