A tank pump assembly comprising a liquid tank for receiving a volume of liquid to be pumped and a centrifugal pump, the liquid tank surrounding a pumping chamber of the centrifugal pump, the centrifugal pump comprising: said pumping chamber having: an inner surface defining a pump cavity; a pump inlet defined in a first side of the pumping chamber; a shaft opening defined substantially centrally in a second side of the pumping chamber, the second side substantially opposing the first side and arranged to be above the first side, in use; and a pump outlet; an impeller retained within the pump cavity; a shaft member mechanically connected to the impeller through the shaft opening, whereby rotation of the shaft member causes rotation of the impeller about a shaft axis passing through the shaft opening and movement of a pumping liquid from the pump inlet towards the pump outlet, wherein the shaft axis is arranged to be substantially vertical, in use; and a flow modifier fixedly mounted relative to the pumping chamber adjacent to an outer surface thereof at the shaft opening to substantially prevent ingress of air into the pump cavity through the shaft opening during operation of the pump even when a liquid level in the liquid tank surrounding the pumping chamber drops below a level of the shaft opening, wherein the flow modifier comprises: an annular portion having defined therein a further shaft opening spaced from the shaft opening in the second side of the pumping chamber and having the shaft member passing therethrough, wherein the centrifugal pump further comprises a spacing member spacing the annular portion from the outer surface, and the flow modifier defines a liquid overflow outlet for liquid flow from the shaft opening in a direction substantially transverse to the shaft axis, and wherein the liquid overflow outlet is sized to substantially prevent the ingress of air into the pump cavity through the shaft opening during operation of the pump, even when the liquid level in the liquid tank surrounding the pumping chamber drops below the level of the shaft opening in the second side of the pumping chamber. said pumping chamber having: an inner surface defining a pump cavity; a pump inlet defined in a first side of the pumping chamber; a shaft opening defined substantially centrally in a second side of the pumping chamber, the second side substantially opposing the first side and arranged to be above the first side, in use; and a pump outlet; an inner surface defining a pump cavity; a pump inlet defined in a first side of the pumping chamber; a shaft opening defined substantially centrally in a second side of the pumping chamber, the second side substantially opposing the first side and arranged to be above the first side, in use; and a pump outlet; an impeller retained within the pump cavity; a shaft member mechanically connected to the impeller through the shaft opening, whereby rotation of the shaft member causes rotation of the impeller about a shaft axis passing through the shaft opening and movement of a pumping liquid from the pump inlet towards the pump outlet, wherein the shaft axis is arranged to be substantially vertical, in use; and a flow modifier fixedly mounted relative to the pumping chamber adjacent to an outer surface thereof at the shaft opening to substantially prevent ingress of air into the pump cavity through the shaft opening during operation of the pump even when a liquid level in the liquid tank surrounding the pumping chamber drops below a level of the shaft opening, wherein the flow modifier comprises: an annular portion having defined therein a further shaft opening spaced from the shaft opening in the second side of the pumping chamber and having the shaft member passing therethrough, wherein the centrifugal pump further comprises a spacing member spacing the annular portion from the outer surface, and the flow modifier defines a liquid overflow outlet for liquid flow from the shaft opening in a direction substantially transverse to the shaft axis, and wherein the liquid overflow outlet is sized to substantially prevent the ingress of air into the pump cavity through the shaft opening during operation of the pump, even when the liquid level in the liquid tank surrounding the pumping chamber drops below the level of the shaft opening in the second side of the pumping chamber.