Unions have blasted employment minister Michaelia Cash for refusing to commit to fix an industrial relations bill that would require them to meet higher disclosure requirements with retrospective effect.

The Australian Council of Trade Unions has warned if passed in its current form the bill would hold unions in breach for failing to report to the Registered Organisations Commission in 2014 and 2015 despite the fact it does not yet exist.

It comes on top of a growing controversy that the building code attached to the Australian Building and Construction Commission bill will also have retrospective effect.

The registered organisations bill, which passed the lower house last week, has a commencement date of 1 July 2014. If the Senate passes it without amendment it will have retrospective effect.

The bill increases requirements for union officials to disclose material personal interests and for unions to report their financial accounts. It increases civil penalties and introduces criminal offences for serious breaches of officials’ duties.

In Senate estimates last Wednesday, Cash said the bill has not been amended because it had been a double-dissolution trigger and therefore needed to be reintroduced in identical form.

She said the government would consider technical amendments if they were “necessary”.

But after repeated requests to clarify whether the government would make amendments to prevent its retrospective effect, a spokesman for Cash said only that the Senate could move amendments without compromising the ability to take the bill to a joint sitting.

It is the second dispute about retrospectivity of elements of the government’s industrial relations agenda. The building code attached to the ABCC bill affects all deals struck after 24 April 2014 and will therefore have retrospective effect.

The code will bar construction companies winning government work because of the content of industrial agreements they struck before it had been passed.

Cash has claimed the code is prospective in effect because it will only bar access to future government projects but has not said how companies with non-code compliant agreements can be expected to comply.

Despite the minister’s claim, major law firms including Corrs Chambers Westgarth are already advising the code will “retrospectively apply to enterprise agreements made after 24 April 2014”.

The ACTU secretary, Dave Oliver, told Guardian Australia the refusal to commit to amend the registered organisations bills was “yet another example of the toxic combination of fanaticism and incompetence that has plagued this government”.

“It beggars belief that the minister will not rule out the possibility that this bill will pass and hold every union in breach of the law for not reporting to an organisation that did not exist,” he said.

“It seems that the minister doesn’t know or doesn’t care what impact her own legislation will have on working people.”

The shadow employment minister, Brendan O’Connor, said Cash “must explain whether this is a proposed retrospective law or another mistake from this lazy and incompetent government”.

“Will this mean that all sanctions are retrospective? Will this mean disclosure requirements are retrospective and impossible to comply with?”

Employment law expert and Adelaide law school professor, Andrew Stewart, said the retrospective effect of the building code was an even greater problem that was causing major concern in the construction industry.

“Virtually every major builder will be noncompliant,” he said, because of the broad range of deal content barred by the code and its retrospective effect.

He said the code could be changed to only bar agreements struck after it passed, in which case it would have “minimal practical effect”, or it would apply retrospectively and “force building companies to renegotiate their enterprise agreements simultaneously causing industrial mayhem”.

“When the government has to change [the code], and will have to change it, how will they do it in a way that doesn’t remove any practical effect for this whole term of government?”

Stewart said the code barred “any clauses with any impact on productivity, or the right of companies to manage their business”.

“It essentially gives [Fair Work Building and Construction director] Nigel Hadgkiss or whoever is the director of the ABCC discretion to object to any union agreement on a very wide number of bases.”