There is one main reason why any business needs an A.B.N.:
Under the Tax laws if a company does business with another business or contractor which does not quote an A.B.N. on their invoice the paying Company is obliged to withhold the G.S.T. amount and account to the Tax Department for it in much the same way as they handle employee P.A.Y.E. deductions. Most of them can't be bothered with this so adopt a policy of no A.B.N., no business.
The only exemption is if a contractor's turnover is under $50,000pa they can opt out of paying G.S.T. but, of course, they cannot claim G.S.T. credits for their purchases either.
You need advice from an Accountant to decide how best to structure your business.