The idea of using staple foods for communion has been around since the beginning of Christianity. That is, apparently, one of the reasons bread and wine were chosen by the early Christian churches. These were relatively cheap and common staples for Mediterranean-rim communities. Commonness speaks of the very ordinary places in which God chooses to dwell and act. Good theology.
It should be remembered, though, that (unleavened) bread and wine were not chosen only because they were common. They were chosen because of their particular Jewish history as symbols of exodus and of atoning sacrifice. The (not yet risen) bread reminded people of the haste with which they fled the oppressor. The wine reminded them of the blood of the lamb by which the Angel of God's wrath recognised their homes and passed over or by.
The early Christian communities also learned from Jesus that the bread and wine were to symbolise his body and his blood at their ritual meals, a body broken and blood poured out in atoning and liberating sacrifice. Wine was chosen not simply because it was common, but because it was red like Christ's blood, and because it was a drink of celebration already associated with the salvation history of the Exodus.
For that reason, I find it rather difficult to accept that common billy tea could really function to carry all those meanings. It is not red and, as far as I know, carries no liberative or salvific meanings in either Indigenous or migrant Australian cultures. That said, I'd be happy to consider the use of other red-coloured drinks such as some Indigenous Christian communities actually do - some of them derived from native plants - but not common billy tea.
One other reason I'd balk at using tea is because of its colonial history. It was very often one of the substances which colonial authorities used to 'buy' Aboriginal land. It was very often exchanged for land, at least in the understanding of whitefellas. For that reason, tea is not a neutral pan-Australian symbol. It is one of the instruments by which the country was stolen. I have a few misgivings about the use of damper in Indigenous contexts as well, since flour was also one of those colonial buying tools. I am not as concerned about this as about tea, however, because flour can at least keep you alive by providing nutrition - and it did keep many Aboriginal communities alive as more traditional food sources were driven away or destroyed. Tea, on the other hand, had and has very little nutritional value. But there are Indigenous alternatives here too, and they are as various as the clans and where they come from. I am a supporter of moves in every community to use whatever is the basic staple at communion [And what is bread, anyway, if not the staple food in any given culture?]
Some have argued for the use of tea on other grounds. Tea can be seen, for example, as a symbol of hospitality, welcome, and an open table. I would agree. In Christ we learned, of course, that God is a hospitable God who would ultimately long to welcome all people to the banqueting table of heaven.
My difficulty with using tea remains, however, because surely the symbols we use at communion need to carry ALL the meanings associated with the meal, and not simply SOME of them. While tea can indeed speak of God's hospitality (in some cultural contexts) it cannot, I would argue, carry the crucial meanings of reconciliation through atonement and of God's sacrificial, costly, love - themes that stand at the heart of the Christian message.
I also have a difficulty with any theology of Eucharist that sees the table of communion as open to absolutely everyone, without remainder. From the beginning, Christians certainly welcomed everyone to their ordinary meal tables, whatever their beliefs or lifestyles. Here they followed the example of Christ himself. But they did not welcome everyone to the ritual meal known as the Lord's Supper or the Eucharist. This meal was reserved for the baptised, for those who had 'signed up', as it were, to the Christian life - with all its beliefs and practices. Why? Because the meal was seen as a weekly reaffirmation of the covenantal promises made in baptism. Now, you can't RE-affirm what you've never affirmed in the first place. In that context, it made no sense to welcome those who were not signed-up. And it still doesn't.
So the invitation to the table is indeed for all. But the mode by which Christ's invitation may be accepted is by passing through the waters of baptism, which (in Christian understanding) is our death to the basic principles of this dark age, and our rising with Christ to a new (de-colonised) way of life.
Let me conclude by noting that the use of tea instead of wine (or another blood-coloured drink) is not something that has been proposed or practiced at any official Uniting Aboriginal and Islander Christian Congress gathering that I have attended. Certainly not at the national theological forum we held about worship and the Eucharist in Jabiru during 2010. As one of the Aboriginal theologians helping to form both policy and practice on these things, I would strongly resist any such move.