Australia’s three major political parties failed to declare sizeable donations from corporate interests seeking lucrative government work, mining approvals or favourable tax policies, the Guardian can reveal.

The Liberals failed to declare a $10,000 donation from Raytheon, an arms manufacturer that was at the time vying for defence contracts, including on Australia’s major shipbuilding and submarine projects. The party’s South Australian branch has blamed the failing on a “clerical error”.

The West Australian Nationals failed to declare a $20,000 cheque it was handed by Mineral Resources, an iron ore miner seeking government approval to access new deposits in environmentally sensitive and unique mountain ranges in the state’s Yilgarn region.

Federal Labor failed to properly disclose a $100,000 donation from the car salary packaging industry, received the same financial year the opposition leader, Bill Shorten, wrote a letter to the industry, pledging to maintain generous tax arrangements.

Labor did make some attempt to disclose the money, but wrongly classed what was a donation from the industry as an “other receipt” in its books. A Labor spokeswoman also blamed the failing on a “clerical error” and said it was addressed soon after the party was notified.

All three donations were received in 2016/17. The donations were not declared until September and November 2018, after probing by the Australian Electoral Commission. In the case of Labor and the Nationals, the AEC picked up the mistakes after noticing discrepancies in the declarations made by donors and parties. The missing Raytheon donation to the Liberals was detected during an AEC compliance review.

Experts say the cases expose the weaknesses in Australia’s donation disclosure regime.

Omissions are not picked up unless a review is conducted by the AEC, and even then it can take more than a year for them to be disclosed, by which time their relevance has been lost.

There is often little direct punishment for the parties, according to the Grattan Institute researcher Carmela Chivers, who worked on a major report on untraceable donations earlier this year.

“It’s tricky because there’s a real balancing act that the AEC needs to walk,” Chivers said. “On the one hand, if you don’t have penalties for non-disclosure, then why would the parties follow the rules?”

“But on the other hand, if the penalties are too severe, then the parties have no interest to amend the forms at all.”

The Liberals’ South Australian branch also failed to declare $20,000 in “other receipts” from Raytheon, on top of the missing $10,000 donation.

Political parties were only required by the AEC to disclose donations above $13,200 in 2016-17. But the AEC told the Liberals they should disclose the donation and the “other receipt”, and the Liberals have conceded the “omissions” were made in error.

The party’s SA state director, Sascha Meldrum, said the missing Raytheon donation was quickly addressed.

“In May this year the AEC conducted a review into the SA division’s return in the 2016-17 financial year,” Meldrum said. “An AEC report was released this month recommending an amended return be submitted, which was promptly acted on. The omissions were due to a clerical error, which was corrected as soon as it was brought to our attention.”

Raytheon did not respond to questions.

The Nationals’ WA branch similarly failed to respond when asked why it failed to declare the $20,000 donation from Mineral Resources.

The iron ore miner confirmed it had given a $20,000 bank cheque to the Nationals in March last year, around the time of the state election. But a spokesman said the company made donations to all sides, not just the Nationals. Its donations to Labor and the Liberals were declared properly.

It denied any suggestion that its donation was linked to a controversial bid to access two new deposits in the Helena-Aurora Range, a unique banded ironstone range declared in 2007 as “one of the more significant biodiversity assets in WA”.

The environmental protection agency had previously refused to even consider the bid because it was environmentally unacceptable. The then-Liberal government intervened and forced an EPA assessment.

Labor subsequently won office in the 2017 WA election and the expansion project was blocked late last year.

A spokesman for Mineral Resources said it was in no way trying to influence the process through its donations.

The National Automotive Leasing and Salary Packaging Association (Nalspa) gave its $100,000 donation to Labor in 2016-17. The association represents businesses involved in employer-provided car salary packaging, and is a significant donor to both Labor and the Liberals.

In 2013, Labor threatened to cut tax breaks benefitting the industry. But the opposition changed its stance in the lead-up to the 2016 election.

In May 2016, Shorten wrote a letter to Nalspa members, saying a Labor government would “retain the current arrangements in relation to all measures regarding salary packaging and related Fringe Benefit Tax measures” and “not implement any changes to the Statutory Formula Method relating to employer provided motor vehicles”.

The announcement caused a jump in the stocks of car salary packaging businesses.

A Labor spokeswoman said the error classifying the money from Nalspa as an “other receipt” rather than a donation was addressed quickly.

“The ALP has always, and continues, to fully comply with electoral law,” she said. “Our standards go beyond what is required by law, with a self-imposed policy of disclosing any federal donation above $1,000.”

Last week, the Senate passed donations reforms designed to ban foreign political donations. The Greens had urged the government to use the bill to implement sweeping reforms to the donations and disclosure systems. Those calls went unheeded.

In June, a Greens-dominated inquiry into donations recommended bans on donations from developers, banks, mining companies and the tobacco, liquor, gambling, defence and pharmaceutical industries. It also called for the continuous real-time disclosure of donations to the AEC, something already occurring at a state level in some jurisdictions, including Queensland.

Real-time donation disclosure systems would help to quickly identify discrepancies between what is declared by a party and a donor, according to the Grattan Institute.