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Australia's economic engine has sputtered as the commodities boom loses steam, but the country could get a fillip from legalizing same-sex marriage. Malcolm Turnbull's ousting of Tony Abbott as prime minister last month sparked speculation the issue could be revisited: Turnbull has made his pro-marriage equality views known. So although Turnbull has said he would stick to his predecessor's timetable of holding a referendum on the issue sometime after the next election -- which must be held by January 2017 -- calculations are already being made on how same-sex marriages could benefit Australia. "In an economy that is challenged for fresh sources of domestic demand, a rush of same-sex weddings would have a small positive impact on activity and confidence," ANZ said in a note Tuesday.

"There would be an economic benefit from increased expenditure on weddings, honeymoons, and the potential improved image of Australia as a more tolerant and progressive country." Read More RBA trims Australia's growth outlook

Australia's Marriage Act currently specifies marriage must be heterosexual, although some states allow same-sex partnerships, which provide access to some, but not all, of the government services available to heterosexual couples.

ANZ estimates the additional expenditure on related to weddings alone would be at least 500 million-550 million Australian dollars ($354 million-$389 million) in just the first year, assuming that half of the current same-sex couples marry within 12 months of legalization. To be sure, that figure is modest for a near $1.5 trillion economy, but ANZ's calculations do not yet include the impact of a favorable decision on travel both in and out of Australia. Read More What a new PM could mean for Australia's economy

"This figure does not include honeymoon expenditure, which would add to the stimulus – especially if foreign couples come to Australia to marry or if Australians are encouraged to stay at home (a trend that may be encouraged by the depreciating Australian dollar)," ANZ said. The economy is badly in need of fresh engines as a slowdown in China has dented demand for commodities, among Australia's primary exports. Quarterly economic growth fell to a two-year low at 0.2 percent during the April-June period, slowing from 0.9 percent in the previous quarter, fueling predictions Australia may soon experience its first recession in more than two decades. ANZ's estimates of the economic boost aren't terribly out of line with figures in the U.S. In an analysis issued in December of last year, before the U.S. Supreme Court struck down same-sex marriage bans in June, the Williams Institute at the UCLA School of Law, in partnership with Credit Suisse, estimated that legalization would unlock around $2.6 billion in a same-sex wedding boom in the first three years, with around $733 million coming from sates in the South and Midwest.