The Government has renewed an offensive to create a transtasman drug agency, but is likely to have to explain an embarrassing defeat to the Australians.

The two countries have been working on the plans for the joint regulatory regime since the late 1990s and signed a treaty in 2003.

The agency was supposed to be established next month but, struggling for votes, State Services Minister Annette King announced a deferral in December.

Under the new timetable the Government aims to introduce the legislation next month and establish it mid next year.

In recent weeks it has embarked on a lobbying campaign of parties it hopes might come onside, including New Zealand First and the Maori Party.

Last night Health Ministry officials undertook a cross-party briefing of MPs as the Government intensifies its efforts.

But unless there are significant changes to the proposed plan, the framework of which was long ago determined between the two Governments, there appears little chance of it succeeding.

The Maori Party is the only party yet to decide how to vote - but could not give the Government the numbers it needs to introduce the law.

The Greens, New Zealand First, Act, United Future and National are all opposed to the plan, with only some prepared to waver if there were significant changes.

Most of the parties said as much in a select committee report written in 2004 and say "draft rules" put out for consultation last month reveal little deviation from the original proposal.

Mrs King refused to comment on the issue yesterday, other than to confirm officials were briefing MPs.

The Australian Government is due to release a draft of its bill next month - which would appear to be pointless if Mrs King cannot get the support.

Associate Health Minister Peter Dunne confirmed yesterday that he had told a select committee last week he didn't believe the Government currently had the numbers.

National's health spokesman, Tony Ryall, said the party had long made its concerns about the agency clear.

These included "the compliance costs and the effects on natural products and the effect that might have on small businesses and consumer choice".

National was in Government when the agency was proposed and Mr Ryall said "we've always said it had some merit, but the devil was in the detail".