Australia's promising start to the deciding Test against New Zealand soured after just nine overs today when paceman Glenn McGrath left the field with a back strain.

At lunch the Kiwis were 2-87, with captain Stephen Fleming and debutant Lou Vincent contributing an unbeaten 33 runs each in a partnership of 68.

After a bright start, the Australians' mood matched the steadily darkening skies above the WACA as McGrath (1-15) was taken to hospital for a precautionary x-ray.

With McGrath absent, Australian skipper Steve Waugh was forced into the unusual step of calling on part-timer Damien Martyn (0-11) in the opening session on the WACA's pace friendly wicket.

If ruled out of taking any further part in the third Test, his loss would leave Australia with Jason Gillespie (1-17) and Brett Lee (0-29) as the only pace options to join leg-spinner Shane Warne (0-8) in the front line against New Zealand's determined line-up.

It would also be a huge blow to Australia's chances in the upcoming three-Test series against South Africa starting at the Adelaide Oval on December 14.

After winning the toss and electing to bat, the Kiwis were in trouble at 2-19 after just half an hour of play before Fleming and Vincent steadied the ship.

Vincent struggled early and was beaten consistently by McGrath in the opening overs before settling in and batting for two hours to survive his first session in the top flight.

Gillespie made the breakthrough in the sixth over when, bowling into a stiff south-easterly breeze, he found an avenue through Mark Richardson's defence and rattled the left handed opener's stumps.

McGrath struck just seven runs later when he trapped Mathew Sinclair in front for two.

© 2001 AAP