Federal frontbencher Christopher Pyne weighed in on Monday, attacking Labor over its criticism of Opposition Leader Steven Marshall.

Mr Pyne's comments came as Mr Marshall faced continued questions over a possible $1.2 million donation to the Liberals from Chinese businesswoman Sally Zou.

Ms Zou tweeted out a picture of a cheque made out to the party on Friday, but Mr Marshall said no such donation had been received.

Mr Pyne said the businesswoman was not the focus of the campaign and argued Labor was just trying to draw attention away from its 16-year, scandal-plagued government.

"This is Labor just trying to smear and distract," Mr Pyne told reporters.

Mr Marshall said any donations received by the party to date had been declared according to the rules.

"The Liberal Party's policy agenda will never, ever be available to be bought," he said.

But Treasurer Tom Koutsantonis continued Labor's call for Mr Marshall to reveal if the $1.2 million cheque had been received and whether or not the Liberals would be cashing it in.

"He should commit to telling South Australians the moment it arrives, rather than disclosing it after the election," Mr Koustantonis said.

Mr Marshall did some political point scoring of his own on Monday, appearing at a federal government announcement of a $1.2 billion defence contract to upgrade the nation's over-the-horizon radar system.

No Labor representative was present at the event, though it remained unclear if any were invited.

With less than two weeks to polling day, Nick Xenophon also questioned the tactics of the Greens to preference Labor and Liberal candidates before members of his SA-BEST team in at least two-key seats.

He called on Greens' voters to ignore the how-to-vote card after a Conservation SA survey recently rated SA-BEST policies on the environment better than those of both Labor or the Liberals.

"The Greens Party decision defies principle and logic. It's incomprehensible and unprincipled," Mr Xenophon said.

"They are siding with the Liberals, many of whom are still captured by the coal lobby."

South Australians go to the polls on March 17, but early voting opened on Monday.

More than 90,000 people cast an early ballot in 2014 and that number is expected to increase this year meaning their decisions could be decisive in a number of key seats.