I want the company to have total authorised capital of $1,000, and issue 550 now, and 450 at a later date. Do I need to add a clause in the constitution to cover this?
There is no need for an additional clause. The concept of authorised capital was abolished in 1998, by the Company Law Review Act 1998. Under our existing constitution, the company effectively can issue an unlimited number of shares at any time in the future, and "the unissued shares in the Company are under the control of the Directors". That is, the directors have the power to issue new shares, and to determine the rights that will attach to the new shares (they can do this by, and record it in, a future resolution of the directors).