How Commonwealth reciprocal aid worked across separate accounts

Did Australia and New Zealand provide more support to American forces than they received from the United States, and was that aid simply buried in Britain's account?

They could provide more than they received. Each was a separate signatory, although integrated forces created some legitimate overlap in the accounts.

Australia, New Zealand, Canada, and Britain were separate signatories within a wider mutual-aid network. Australia and New Zealand could supply American forces with more than they received without folding those contributions into a single British bill. Integrated forces still created legitimate accounting overlap when one Dominion's resources supported a formation recorded under another government's account. British jet-engine plans are one example of technical information transferred without an ordinary sale. The operational principle was to provide what an ally needed without delaying over payment.