A bill overhauling the health and safety laws in New Zealand in the wake of the Pike River disaster has been delayed because of concerns within the National Party caucus about its effect on small businesses and farmers.

Prime Minister John Key confirmed it had been the subject of discussion at yesterday's caucus meeting and said he wanted to have more time to get it right.

"It's started to get panel beaten into quite good shape."

The Health and Safety Reform Bill was due to be reported back this week from the transport and industrial relations committee but it has now been given until July 24 to report back.

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The main sticking points in the caucus are thought to be:

* Whether businesses with 20 staff or less should be exempt from the requirement to have health and safety representative on a worker's request, as is the case now and in the original bill.

* Whether types of businesses can be deemed exempt from the law at any time by regulation, as the bill allows.

The Minister for Workplace Relations and Safety, Michael Woodhouse, told reporters some of the concerns were about compliance requirements.

Mr Key told reporters he was concerned about making sure the legislation could be implemented by the vast array of businesses across New Zealand, 95 per cent of which were small.

Mr Key said he had put it on the caucus agenda and denied there were divisions about it.

"I wanted to have a bit more time because I wanted to be absolutely sure it's right."

Labour's industrial relations spokesman, Iain Lees-Galloway said he was concerned that the bill could be "watered down to the point of being ineffectual".

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"I am also concerned that we could end up with health and safety legislation that is actually weaker than what we have right now."

Former minister Judith Collins is thought to have been one of those in caucus promoting changes to the bill as well as MPs who have been lobbied by small businesses in their electorates.

The bill was drafted to implement recommendations of the Royal Commission on the Pike River coal mine disaster in which 29 men died, and the Independent Taskforce on Workplace Health and Safety.

Modelled on Australia's health and safety regime, it sets out the duty of care of a person conducting a business of undertaking.

The extension of time may give other parties time to get more changes to the bill as well.