The New Zealand Superannuation Fund will announce in December which investments it is pulling out because of their involvement in the manufacture of cluster bombs.

Fund chief executive Adrian Orr said the divestments would cover a range of small and large companies, making up a very small part of the fund's $14.5 billion worth of investments.

The announcement would coincide with the signing of an international treaty banning cluster bombs.

New Zealand was among 111 countries that in Dublin, Ireland, adopted a draft treaty banning cluster bombs.

A final signing ceremony for the Dublin agreement will take place in Oslo on December 3, but many of the leading users of cluster bombs will not take part.

Mr Orr said it was unusual for superannuation funds to exclude companies and the preference was to work them improve their environmental, social and corporate behaviour.

In the case of cluster bomb production, the treaty and New Zealand's lead in the process, combined with the fact that it was unlikely the companies could be persuaded to stop manufacturing, meant it was best exclude them.

Mr Orr said exclusion would continue to be rare in the future and the fund preferred to work with other funds to pursue responsible investment and persuading companies with poor records to change their behaviour.

He argued that responsible investment patterns would out perform investments in companies.

The fund has previously announced it would pull out of investments in companies involved with landmines, whaling and tobacco.

The fund would also make a decision on investments in companies involved in nuclear weapon manufacturing and was working on developing responsible investment guidelines across all of its portfolio.