N.J. Stat. Ann. § 54A:5-7

Allocations of income of nonresidents. Where a nonresident taxpayer earns, receives or acquires income from sources partly within and partly without this State or engages in a business, trade, profession or occupation partly within and partly without this State, and, as a result thereof or for other reasons that portion of the income from sources within this State cannot readily or accurately be ascertained, the director may by regulation prescribe uniform rules for apportionment or allocation of so much of such taxpayer's income as fairly and equitably represents income, derived from sources within this State and subject to tax under this act.