The benchmark S&P/ASX20 on Monday dropped 13.7 points, or 0.16 per cent, to 8,589.3, while the broader All Ordinaries fell 15.5 points, or 0.18 per cent, to 8,826.4.
Overseas, US stock market futures fell after President Trump confirmed that America's tariffs won't come into effect into August 1, rather than this week, potentially dragging out the drama even further.
The White House said it was sending letters detailing "take it or leave it" tariff rates to a dozen countries - it wasn't clear which.
"Tariffs go into effect August 1. But the president is setting the rates, and the deals, right now," Commerce Secretary Howard Lutnick told reporters as the president nodded in approval.
Markets had previously been operating under the assumption that the tariffs would go into effect on Wednesday, with the expiration of 90-day pause the Trump administration announced back on April 9.
US Treasury Secretary Scott Bessent denied in a television interview that the August 1 date represented a new deadline, but it potentially does give America's trading partners more time to negotiate - as well as drawing out the drama further.
Closer to home, the Reserve Bank is widely expected to trim interest rates on Tuesday, with futures markets on Friday giving implied odds for a cut of 97 per cent.
The Aussie dollar had fallen sharply amid the tariff uncertainty and ahead of the RBA meeting, trading for a 12-day low of 65.13 US cents, from 65.72 US cents at close of business on Friday.
IG market analyst Tony Sycamore said that whether the Aussie extended Monday's losses or rebounded would depend on the details of Mr Trump's tariff letters, as well as the outcome of Tuesday's RBA meeting.
Five of the ASX's 11 sectors finished higher and five closed lower, with industrials flat.
Utilities was the biggest mover, rising 3.5 per cent as Origin Energy soared 6.8 per cent to a three-week high of $11.55 following a news report over the weekend that UK-based Octopus Energy - which Origin owns a 23 per cent stake in - might spin off its technology business, Kraken Technologies.
In the mining sector, Northern Star had dropped 8.7 per cent to a more than three-month low of $16.80 after Australia's biggest listed goldminer told investors to expect higher costs and lower production in 2025/26.
Northern Star said the higher costs reflected industry-wide inflationary pressure as well as infrastructure and development costs and royalties.
Elsewhere in the sector, Fortescue was basically flat at $16.34, Rio Tinto dipped 0.2 per cent to $108.62 and BHP retreated 0.3 per cent to $38.60.
In the heavyweight banking sector, all of the big four banks finished lower.
CBA slipped 0.1 per cent to $178.81, ANZ lost 0.6 per cent to $30.13, Westpac retreated 0.5 per cent to $33.48 and NAB slid 0.3 per cent to $39.04.
Elsewhere, Cobram Estate Olives rose 13.5 per cent to an all-time high of $2.44 after the olive-grower reported a successful 2025 harvest.
AML3D gained 4.2 per cent to 25 cents after the US Navy confirmed its interest in purchasing more parts from the industrial 3D metal printing technology company.
ON THE ASX:
* The benchmark S&P/ASX200 index finished Monday down 13.7 points, or 0.16 per cent, at 8,589.3
* The broader All Ordinaries dropped 15.5 points, or 0.18 per cent, to 8,826.4
CURRENCY SNAPSHOT:
One Australian dollar buys:
* 65.13 US cents, from 65.72 US cents at 5pm on Friday
* 94.43 Japanese yen, from 94.58 Japanese yen
* 55.35 euro cents, from 55.75 euro cents
* 47.80 British pence, from 48.14 pence
* 107.26 NZ cents, from 108.21 NZ cents
