There are a few Public Trees on ancestry.com which list Alexander Halley and Margaret McKay. But either there is another couple with those names, or tree owner is in error.
Did your couple have a daughter named Annie, born 1846 Geelong, married William Henry Wright Wilson, died 1914 Geelong? If so, at least this entry is correct.
But this tree has Alexander Halley born Hertfordshire, England in 1856! (Sounds like the same person who attached my g-grandmother's children to another woman with the same forename - remarkably she was married five years before she was born. And then got really upset with me when I messaged her telling her of the errors! But at least she took my mother off - before blocking me from her tree.)
Another tree gives the correct year of marriage (1845) to Margaret Mckay, and Alexander's death in Geelong on 18 May 1920. It also gives Margaret's place of death as Geelong, but no date. Is this correct? According to this tree, Margaret was born 1825 Hertfordshire, England - but no parents.
I don't think any of the trees are correct - they seem to be a meld of information. But there might be a few factual snippets there in there somewhere.
There was an Alexander Halley, aged 21, arrived Geelong on 22 March, 1845, on Sir George Seymour, ex Pentonville Probationary Prison.
There are several Halley burials listed on Geelong Cemetery site,
http://www.gct.net.au/search.html, including Alexander, 18 May 1920, COEO*B***145. In the same plot are Jessie Halley, 10 May, 1934 and Mons Alexander (Is Mons a name, or does it mean something else?), 18 January, 1915. Are these your people?
Cheers
Barbara