Trade unions do not want to be exempted from a potential lobbying register, despite Labour's attempts to have them excluded.

Green MP Holly Walker has a member's bill that would create a register of lobbyists and require them to follow a code of ethics.

The bill passed its first reading with unanimous support, but Labour has since suggested an amendment that would exempt trade unions.

Fairfax Media can now reveal that the unions have not asked for that and say it would not be practical.

In a draft submission, the Council of Trade Unions expressed concern about the proposed regime.

"Lobbying is a core activity of trade unions, but the consequences on trade unions and NGOs from this bill if it were to proceed have not been sufficiently considered."

There were practical difficulties in establishing which staff within a union would have to be registered and the reporting requirements and penalties were "particularly burdensome".

But it did not ask to be exempted from the scheme "on the basis of being unions".

Rather, the CTU sought different reporting requirements for unions and non-governmental organisations.

"The CTU has concerns about the increasing influence of corporate lobbying and the influential impact of professional and secret lobbying."

Scandals of secret and powerful lobbyists overseas and the growing influence of corporate lobbyists here were behind the proposed law, the submission said.

However, Ms Walker's bill should not be rushed through without full consideration of its complexities and flow-on effects.

"The bill in its current form treats all lobbyists as if they are equal when lobbyists' power and influence is very unequal."

Earlier this week Labour MP Trevor Mallard said the bill would capture a union official who rang him to help with a constituent's housing problem.

The Government administration select committee is due to report back on Ms Walker's bill by the end of January.

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