Angus Taylor’s department ignored advice warning not to run a $2.43m taxpayer-funded ad blitz spruiking the government’s energy policies in the lead-up to Christmas, internal documents show.

The multimillion-dollar “Powering Forward” campaign boasted of the government’s efforts to bring down power prices, using ads in newspapers and on digital news sites, social media, television and radio.

The campaign was spread across five phases and used language that closely reflected the Coalition’s political messaging, including phrases such as “we’ve turned the corner on energy ­prices”, which Labor said had been used regularly by government MPs. Crossbench MPs have separately complained that ad campaigns such as Powering Forward represent an enormous expense to taxpayers, without any real assurance of their effectiveness.

Documents obtained by Guardian Australia under freedom of information show the government’s advertising contractor, UM, spent about $2.43m on media for Powering Forward in December, when the fourth phase of the campaign ran.

A separate FOI request shows UM had previously warned against running the fourth phase in December, because audiences would have little interest in energy issues in the lead-up to Christmas.

“Consumer interest and information seeking activity surrounding the energy category begins to decline from mid-November with further significant declines seen from early December reaching the lowest point on Christmas week,” UM said in a strategy document dated 12 November.

“Search behaviour returns to a more consistent level from the end of January. To capitalise on the natural behavioural trend surrounding energy category engagement, UM would recommend launching phase four in late January or early February 2019.

“Upon advice from the department, activity has been planned to run from 2 to 15 December 2018.”

The department denied it had ignored advice from UM. It said UM had recommended not running the campaign past 15 December, which it adhered to.

Government advertising campaigns are required not to be “conducted for party political purposes”. Campaigns worth more than $250,000 are subject to added scrutiny by an independent communications committee, which found earlier this year that Powering Forward was not in breach of the requirements.

The Labor MP Pat Conroy, then the shadow assistant energy minister, last year referred the campaign for audit, saying the slogans used in Powering Forward were regularly used by senior members of the Coalition.

“I ask that you consider whether the use of the slogan ‘We’ve turned the corner on electricity prices’ constitutes a breach of principle 3 of the guidelines, given the slogan is attributable to members of the government, including former prime minister, Malcolm Turnbull, who repeatedly used this phrase during his prime ministership,” Conroy wrote.

“Close variants of this slogan have been used by the current prime minister, Scott Morrison, the current treasurer and former energy minister, Josh Frydenberg, and the current energy minister, Angus Taylor.

“These comments are neither ‘objective’ nor free of ‘party political’ themes, and have been at the centre of the Liberal party’s political strategy.”

Crossbench MPs, including Greens MP Adam Bandt and the Centre Alliance’s Rebekha Sharkie, made a separate request earlier this year that all government advertising be audited.

They feared huge amounts were being spent on government advertising with no real measure of whether it was working.

“When substantial public funds are spent with no assessment of effectiveness, it begs the question: is this really an appropriate use of taxpayer money?” the crossbenchers wrote in April.

Conroy told Guardian Australia the government was simply rolling out “millions of dollars in propaganda, ignoring advice from the government’s own advertising experts on the campaign”.

“This is yet more evidence that the Morrison government’s Powering Forward advertising campaign is a politically-motivated misuse of taxpayer funds rather than a genuine information campaign.”

The Australian National Audit Office is considering the issues raised by Conroy and the crossbenchers as part of a broader audit of government advertising, due to be completed this month.

The audit is examining Powering Forward and separate campaigns run by the Department of Education and Training, and the Department of Health, whether the finance department “effectively administered the advertising campaigns” and whether they complied with government advertising requirements.

