The Australian-English phrase as Australian as meat pie and its variants mean typically Australian in character.

It is an alteration of the phrase as American as apple pie, with reference to the prominence of meat pie in Australian diet.

These are the earliest occurrences of as Australian as meat pie that I have found, in chronological order:

1-: From Show Business: Valda Marshall talks about TV, published in The Sun-Herald (Sydney, New South Wales) of Sunday 10^th April 1966—Rolf Harris (born 1930) is an Australian entertainer:

Rolf is a unique entertainer. Without any “gee, it’s wunnerful to be back” routine, and with a voice that’s as Australian as a meat pie, he captures an audience single-handed within minutes.

2-: From an advertisement for Renault (Aust.) Pty. Ltd., published in The Bulletin (Sydney, New South Wales) of Saturday 4^th March 1967:

When you buy a factory in West Heidelberg, Victoria and employ 340 Australian craftsmen and executives, you quickly become assimilated (As Australian as meat pie and Tomato Sauce). More important you make cars that master local conditions.

Be rewarded—Drive it!

3-: From Gordon Chater—man of a thousand faces, by Garry Shelley, published in The Sydney Morning Herald (Sydney, New South Wales) of Monday 27^th December 1971—Gordon Chater (1922-1999) was an Australian actor:

Being as Australian as the meat pies he has devoured most inelegantly in the Bramston shows ¹, it seems only natural that Gordon Chater should have been selected to star in the pilot of ATN 7’s planned series Snake Gully—based on the long-running radio serial Dad and Dave ².

¹ The Mavis Bramston Show (1964-68) was an Australian television satirical sketch comedy series.
² Snake Gully with Dad and Dave (1972) was an Australian television series adapted from the radio series Dad and Dave from Snake Gully (1937-53).

4-: From Soap operas, by Don Anderson, published in The Bulletin (Sydney, New South Wales) of Saturday 13^th May 1972—John Meillon (1934-1989) was an Australian actor:

Like all bad soap opera, it [= “Lane End”] displays a radical failure of intelligence. If you can’t cope with character, you slip into caricature; if you can’t dramatise problems, you descend to the sentimentalising of Big Issues.
Yet, despite a poverty of ideas, wooden-legged scripts, execrable acting and insensitive direction, the series is redeemed. How? By the brilliant performance of John Meillon as Ray Dunlop, a Ron Hodgson of a car dealer. Meillon gets and delivers great lines—but then, the Ron Hodgson ads, on which they seem to be based, are among the best written material on Australian TV.
I am drawn back to “Lane End” each week to see Meillon, his face as genuinely Australian as a meat pie, give his performance. Of course it’s a cadenza, but as Dwight MacDonald observed of Peter Sellers’ performance in Kubrick’s film of “Lolita”, who doesn’t admire a great cadenza? It’s a pity it’s set in such a vile concerto.

5-: From New in New York, by Kerry Myers, published in The Sun-Herald (Sydney, New South Wales) of Sunday 20^th May 1973:

Excitement is growing in New York this week as the time for the play-off for the “Balzary Cup” approaches.
If you haven’t heard of the Balzary Cup you should be ashamed of yourself as we hear it is as Australian as meat-pie’n sauce.
Actually this staffer heard of it for the first time this week when an invitation from the Australian Consulate-General in New York arrived on his desk.
“The winner of this prestige championship (golf by the way) so rich in honour and tradition . . .” read my invitation.
“. . . can justly lay claim to the title of the Australian amateur golfing champion (Northern Hemisphere division).” And that’s not all.
“. . . At the conclusion of the championship an informal barbecue (supply your own) will be held at which time, it is hoped, the Consul-General will make the presentation.”

6-: From The Jack Egerton story, by Alan Reid, published in The Bulletin (Sydney, New South Wales) of Saturday 16^th October 1976—ALP is the abbreviation of Australian Labor Party:

Blunt as the rear end of a timber cutter’s axe, bush-raised, as unashamedly Australian as the meat pie, earthy but perceptive, Queenslander Jack Egerton, 58, boilermaker, square-built, pot-bellied and pungent of speech, was for years part of the leadership of the Australian industrial movement and the ALP. He was president of the Queensland Trades and Labor Council and senior Federal vice-president of the ALP and on occasions acted as ALP Federal president.